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_mapper.py

@@ -304,7 +304,7 @@ async def test_executor_completed_event_with_agent_response(
 
     This is a REGRESSION TEST for the serialization bug where
     WorkflowEvent.data contained AgentExecutorResponse with nested
-    AgentResponse and ChatMessage objects (SerializationMixin) that
+    AgentResponse and Message objects (SerializationMixin) that
     Pydantic couldn't serialize.
     """
     # Create event with realistic nested data - the exact structure that caused the bug
@@ -579,13 +579,13 @@ async def test_workflow_output_event(mapper: MessageMapper, test_request: AgentF
 
 async def test_workflow_output_event_with_list_data(mapper: MessageMapper, test_request: AgentFrameworkRequest) -> None:
     """Test output event (type='output') with list data (common for sequential/concurrent workflows)."""
-    from agent_framework import ChatMessage
+    from agent_framework import Message
     from agent_framework._workflows._events import WorkflowEvent
 
-    # Sequential/Concurrent workflows often output list[ChatMessage]
+    # Sequential/Concurrent workflows often output list[Message]
     messages = [
-        ChatMessage(role="user", contents=[Content.from_text(text="Hello")]),
-        ChatMessage(role="assistant", contents=[Content.from_text(text="World")]),
+        Message(role="user", contents=[Content.from_text(text="Hello")]),
+        Message(role="assistant", contents=[Content.from_text(text="World")]),
     ]
     event = WorkflowEvent.output(executor_id="complete", data=messages)
     events = await mapper.convert_event(event, test_request)