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/github_copilot/tests/test_github_copilot_agent.py

@@ -11,8 +11,8 @@ from agent_framework import (
     AgentResponse,
     AgentResponseUpdate,
     AgentThread,
-    ChatMessage,
     Content,
+    Message,
 )
 from agent_framework.exceptions import ServiceException
 from copilot.generated.session_events import Data, SessionEvent, SessionEventType
@@ -290,11 +290,11 @@ class TestGitHubCopilotAgentRun:
         mock_session: MagicMock,
         assistant_message_event: SessionEvent,
     ) -> None:
-        """Test run method with ChatMessage."""
+        """Test run method with Message."""
         mock_session.send_and_wait.return_value = assistant_message_event
 
         agent = GitHubCopilotAgent(client=mock_client)
-        chat_message = ChatMessage(role="user", contents=[Content.from_text("Hello")])
+        chat_message = Message(role="user", contents=[Content.from_text("Hello")])
         response = await agent.run(chat_message)
 
         assert isinstance(response, AgentResponse)