Python: [BREAKING] Simplify API: ChatAgent -> Agent, ChatMessage -> Message (#3747)

* [BREAKING] Rename ChatAgent -> Agent, ChatMessage -> Message, ChatClientProtocol -> SupportsChatGetResponse

Simplify the public API by removing redundant 'Chat' prefix from core types:
- ChatAgent -> Agent
- RawChatAgent -> RawAgent
- ChatMessage -> Message
- ChatClientProtocol -> SupportsChatGetResponse

Also renamed internal WorkflowMessage (was Message in _runner_context) to avoid collision.

No backward compatibility aliases - this is a clean breaking change.

* [BREAKING] Rename Agent chat_client parameter to client

* Fix rebase issues: WorkflowMessage references and broken markdown links

* Fix formatting and lint issues from code quality checks

* Fix import ordering in workflow sample files

* fixed rebase

* Fix test failures: use WorkflowMessage and A2AMessage after ChatMessage→Message rename

- Replace Message(data=..., source_id=...) with WorkflowMessage(...) in workflow tests
- Fix isinstance check in A2A agent to use A2AMessage instead of Message
- Fix import in test_workflow_observability.py (Message→WorkflowMessage)

* Fix lint, fmt, and sample errors after ChatMessage→Message rename

- Auto-fix 70+ ruff lint issues across samples (ChatMessage→Message refs)
- Fix HostedVectorStoreContent→Content.from_hosted_vector_store in file search sample
- Fix _normalize_messages→normalize_messages in custom agent sample
- Fix context.terminate→raise MiddlewareTermination in middleware samples
- Fix with_update_hook→with_transform_hook in override middleware sample
- Add TOptions_co import back to custom_chat_client sample
- Add noqa for FastAPI File() default in chatkit sample
- Fix B023 loop variable capture in weather agent sample

* fix: update Agent constructor calls from chat_client to client in declaration-only tool tests

* fix: add register_cleanup to devui lazy-loading proxy and type stub

* fixed tests and updated new pieces

* fix agui typevar

* fix merge errors

* fix merge conflicts

* fiux merge

* Remove unused links

---------

Co-authored-by: Evan Mattson <evan.mattson@microsoft.com>

python/packages/devui/tests/devui/test_server.py

@@ -142,7 +142,7 @@ async def test_credential_cleanup() -> None:
     """Test that async credentials are properly closed during server cleanup."""
     from unittest.mock import AsyncMock, Mock
 
-    from agent_framework import ChatAgent
+    from agent_framework import Agent
 
     # Create mock credential with async close
     mock_credential = AsyncMock()
@@ -155,7 +155,7 @@ async def test_credential_cleanup() -> None:
     mock_client.function_invocation_configuration = None
 
     # Create agent with mock client
-    agent = ChatAgent(name="TestAgent", chat_client=mock_client, instructions="Test agent")
+    agent = Agent(name="TestAgent", client=mock_client, instructions="Test agent")
 
     # Create DevUI server with agent
     server = DevServer()
@@ -175,7 +175,7 @@ async def test_credential_cleanup_error_handling() -> None:
     """Test that credential cleanup errors are handled gracefully."""
     from unittest.mock import AsyncMock, Mock
 
-    from agent_framework import ChatAgent
+    from agent_framework import Agent
 
     # Create mock credential that raises error on close
     mock_credential = AsyncMock()
@@ -188,7 +188,7 @@ async def test_credential_cleanup_error_handling() -> None:
     mock_client.function_invocation_configuration = None
 
     # Create agent with mock client
-    agent = ChatAgent(name="TestAgent", chat_client=mock_client, instructions="Test agent")
+    agent = Agent(name="TestAgent", client=mock_client, instructions="Test agent")
 
     # Create DevUI server with agent
     server = DevServer()
@@ -207,7 +207,7 @@ async def test_multiple_credential_attributes() -> None:
     """Test that we check all common credential attribute names."""
     from unittest.mock import AsyncMock, Mock
 
-    from agent_framework import ChatAgent
+    from agent_framework import Agent
 
     # Create mock credentials
     mock_cred1 = Mock()
@@ -223,7 +223,7 @@ async def test_multiple_credential_attributes() -> None:
     mock_client.function_invocation_configuration = None
 
     # Create agent with mock client
-    agent = ChatAgent(name="TestAgent", chat_client=mock_client, instructions="Test agent")
+    agent = Agent(name="TestAgent", client=mock_client, instructions="Test agent")
 
     # Create DevUI server with agent
     server = DevServer()