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/orchestrations/tests/test_handoff.py

@@ -7,11 +7,10 @@ from unittest.mock import AsyncMock, MagicMock
 import pytest
 from agent_framework import (
     Agent,
+    BaseContextProvider,
     ChatResponse,
     ChatResponseUpdate,
     Content,
-    Context,
-    ContextProvider,
     Message,
     ResponseStream,
     WorkflowEvent,
@@ -306,16 +305,18 @@ async def test_tool_choice_preserved_from_agent_config():
 
 
 async def test_context_provider_preserved_during_handoff():
-    """Verify that context_provider is preserved when cloning agents in handoff workflows."""
+    """Verify that context_providers are preserved when cloning agents in handoff workflows."""
     # Track whether context provider methods were called
     provider_calls: list[str] = []
 
-    class TestContextProvider(ContextProvider):
+    class TestContextProvider(BaseContextProvider):
         """A test context provider that tracks its invocations."""
 
-        async def invoking(self, messages: Sequence[Message], **kwargs: Any) -> Context:
-            provider_calls.append("invoking")
-            return Context(instructions="Test context from provider.")
+        def __init__(self) -> None:
+            super().__init__("test")
+
+        async def before_run(self, **kwargs: Any) -> None:
+            provider_calls.append("before_run")
 
     # Create context provider
     context_provider = TestContextProvider()
@@ -328,13 +329,13 @@ async def test_context_provider_preserved_during_handoff():
         client=mock_client,
         name="test_agent",
         id="test_agent",
-        context_provider=context_provider,
+        context_providers=[context_provider],
     )
 
     # Verify the original agent has the context provider
-    assert agent.context_provider is context_provider, "Original agent should have context provider"
+    assert context_provider in agent.context_providers, "Original agent should have context provider"
 
-    # Build handoff workflow - this should clone the agent and preserve context_provider
+    # Build handoff workflow - this should clone the agent and preserve context_providers
     workflow = HandoffBuilder(participants=[agent]).with_start_agent(agent).build()
 
     # Run workflow with a simple message to trigger context provider