Python: [BREAKING] PR2 — Wire context provider pipeline, remove old types, update all consumers (#3850)

* PR2: Wire context provider pipeline and update all internal consumers

- Replace AgentThread with AgentSession across all packages
- Replace ContextProvider with BaseContextProvider across all packages
- Replace context_provider param with context_providers (Sequence)
- Replace thread= with session= in run() signatures
- Replace get_new_thread() with create_session()
- Add get_session(service_session_id) to agent interface
- DurableAgentThread -> DurableAgentSession
- Remove _notify_thread_of_new_messages from WorkflowAgent
- Wire before_run/after_run context provider pipeline in RawAgent
- Auto-inject InMemoryHistoryProvider when no providers configured

* fix: update all tests for context provider pipeline, fix lazy-loaders, remove old test files

* refactor: update all sample files for context provider pipeline (AgentThread→AgentSession, ContextProvider→BaseContextProvider)

* fix: update remaining ag-ui references (client docstring, getting_started sample)

* fix: make get_session service_session_id keyword-only to avoid confusion with session_id

* refactor: rename _RunContext.thread_messages to session_messages

* refactor: remove _threads.py, _memory.py, and old provider files; migrate devui to use plain message lists

* rename: remove _new_ prefix from test files

* refactor: rewrite SlidingWindowChatMessageStore as SlidingWindowHistoryProvider(InMemoryHistoryProvider)

* fix: read full history from session state directly instead of reaching into provider internals

* fix: update stale .pyi stubs, sample imports, and README references for new provider types

* fix: remove stale message_store, _notify_thread_of_new_messages, and session_id.key references in samples

* refactor: merge context_providers and sessions sample folders into sessions, remove aggregate_context_provider

* refactor: UserInfoMemory stores state in session.state instead of instance attributes

* feat: add Pydantic BaseModel support to session state serialization

Pydantic models stored in session.state are now automatically serialized
via model_dump() and restored via model_validate() during to_dict()/from_dict()
round-trips. Models are auto-registered on first serialization; use
register_state_type() for cold-start deserialization.

Also export register_state_type as a public API.

* fix mem0

* Update sample README links and descriptions for session terminology

- Replace 'thread' with 'session' in sample descriptions across all READMEs
- Update file links for renamed samples (mem0_sessions, redis_sessions, etc.)
- Fix Threads section → Sessions section in main samples/README.md
- Update tools, middleware, workflows, durabletask, azure_functions READMEs
- Update architecture diagrams in concepts/tools/README.md
- Update migration guides (autogen, semantic-kernel)

* Fix broken Redis README link to renamed sample

* Fix Mem0 OSS client search: pass scoping params as direct kwargs

AsyncMemory (OSS) expects user_id/agent_id/run_id as direct kwargs,
while AsyncMemoryClient (Platform) expects them in a filters dict.
Adds tests for both client types.

Port of fix from #3844 to new Mem0ContextProvider.

* Fix rebase issues: restore missing _conversation_state.py and checkpoint decode logic

- Add back _conversation_state.py (encode/decode_chat_messages) lost in rebase
- Fix on_checkpoint_restore to decode cache/conversation with decode_chat_messages
- Fix on_checkpoint_restore to use decode_checkpoint_value for pending requests
- Add tests/workflow/__init__.py for relative import support
- Fix test_agent_executor checkpoint selection (checkpoints[1] not superstep)

* Add STORES_BY_DEFAULT ClassVar to skip redundant InMemoryHistoryProvider injection

Chat clients that store history server-side by default (OpenAI Responses API,
Azure AI Agent) now declare STORES_BY_DEFAULT = True. The agent checks this
during auto-injection and skips InMemoryHistoryProvider unless the user
explicitly sets store=False.

* Fix broken markdown links in azure_ai and redis READMEs

* Fix getting-started samples to use session API instead of removed thread/ContextProvider API

* updates to workflow as agent

* fix group chat import

* Rename Thread→Session throughout, fix service_session_id propagation, remove stale AGUIThread

- Fix: Propagate conversation_id from ChatResponse back to session.service_session_id
  in both streaming and non-streaming paths in _agents.py
- Rename AgentThreadException → AgentSessionException
- Remove stale AGUIThread from ag_ui lazy-loader
- Rename use_service_thread → use_service_session in ag-ui package
- Rename test functions from *_thread_* to *_session_*
- Rename sample files from *_thread* to *_session*
- Update docstrings and comments: thread → session
- Update _mcp.py kwargs filter: add 'session' alongside 'thread'
- Fix ContinuationToken docstring example: thread=thread → session=session
- Fix _clients.py docstring: 'Agent threads' → 'Agent sessions'

* Fix broken markdown links after thread→session file renames

* fix azure ai test

python/packages/core/tests/openai/test_openai_assistants_client.py

@@ -14,7 +14,7 @@ from agent_framework import (
     Agent,
     AgentResponse,
     AgentResponseUpdate,
-    AgentThread,
+    AgentSession,
     ChatResponse,
     ChatResponseUpdate,
     Content,
@@ -1264,70 +1264,70 @@ async def test_openai_assistants_agent_basic_run_streaming():
 
 @pytest.mark.flaky
 @skip_if_openai_integration_tests_disabled
-async def test_openai_assistants_agent_thread_persistence():
-    """Test Agent thread persistence across runs with OpenAIAssistantsClient."""
+async def test_openai_assistants_agent_session_persistence():
+    """Test Agent session persistence across runs with OpenAIAssistantsClient."""
     async with Agent(
         client=OpenAIAssistantsClient(model_id=INTEGRATION_TEST_MODEL),
         instructions="You are a helpful assistant with good memory.",
     ) as agent:
-        # Create a new thread that will be reused
-        thread = agent.get_new_thread()
+        # Create a new session that will be reused
+        session = agent.create_session()
 
         # First message - establish context
         first_response = await agent.run(
-            "Remember this number: 42. What number did I just tell you to remember?", thread=thread
+            "Remember this number: 42. What number did I just tell you to remember?", session=session
         )
         assert isinstance(first_response, AgentResponse)
         assert "42" in first_response.text
 
         # Second message - test conversation memory
         second_response = await agent.run(
-            "What number did I tell you to remember in my previous message?", thread=thread
+            "What number did I tell you to remember in my previous message?", session=session
         )
         assert isinstance(second_response, AgentResponse)
         assert "42" in second_response.text
 
-        # Verify thread has been populated with conversation ID
-        assert thread.service_thread_id is not None
+        # Verify session has been populated with conversation ID
+        assert session.service_session_id is not None
 
 
 @pytest.mark.flaky
 @skip_if_openai_integration_tests_disabled
-async def test_openai_assistants_agent_existing_thread_id():
-    """Test Agent with existing thread ID to continue conversations across agent instances."""
-    # First, create a conversation and capture the thread ID
-    existing_thread_id = None
+async def test_openai_assistants_agent_existing_session_id():
+    """Test Agent with existing session ID to continue conversations across agent instances."""
+    # First, create a conversation and capture the session ID
+    existing_session_id = None
 
     async with Agent(
         client=OpenAIAssistantsClient(model_id=INTEGRATION_TEST_MODEL),
         instructions="You are a helpful weather agent.",
         tools=[get_weather],
     ) as agent:
-        # Start a conversation and get the thread ID
-        thread = agent.get_new_thread()
-        response1 = await agent.run("What's the weather in Paris?", thread=thread)
+        # Start a conversation and get the session ID
+        session = agent.create_session()
+        response1 = await agent.run("What's the weather in Paris?", session=session)
 
         # Validate first response
         assert isinstance(response1, AgentResponse)
         assert response1.text is not None
         assert any(word in response1.text.lower() for word in ["weather", "paris"])
 
-        # The thread ID is set after the first response
-        existing_thread_id = thread.service_thread_id
-        assert existing_thread_id is not None
+        # The session ID is set after the first response
+        existing_session_id = session.service_session_id
+        assert existing_session_id is not None
 
-    # Now continue with the same thread ID in a new agent instance
+    # Now continue with the same session ID in a new agent instance
 
     async with Agent(
-        client=OpenAIAssistantsClient(thread_id=existing_thread_id),
+        client=OpenAIAssistantsClient(thread_id=existing_session_id),
         instructions="You are a helpful weather agent.",
         tools=[get_weather],
     ) as agent:
-        # Create a thread with the existing ID
-        thread = AgentThread(service_thread_id=existing_thread_id)
+        # Create a session with the existing ID
+        session = AgentSession(service_session_id=existing_session_id)
 
         # Ask about the previous conversation
-        response2 = await agent.run("What was the last city I asked about?", thread=thread)
+        response2 = await agent.run("What was the last city I asked about?", session=session)
 
         # Validate that the agent remembers the previous conversation
         assert isinstance(response2, AgentResponse)