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/ag-ui/getting_started/client_advanced.py

@@ -114,15 +114,15 @@ async def non_streaming_example(client: AGUIChatClient, thread_id: str | None =
 async def tool_example(client: AGUIChatClient, thread_id: str | None = None):
     """Demonstrate sending tool definitions to the server.
 
-    IMPORTANT: When using AGUIChatClient directly (without ChatAgent wrapper):
+    IMPORTANT: When using AGUIChatClient directly (without Agent wrapper):
     - Tools are sent as DEFINITIONS only
     - No automatic client-side execution (no function invocation middleware)
     - Server must have matching tool implementations to execute them
 
     For CLIENT-SIDE tool execution (like .NET AGUIClient sample):
-    - Use ChatAgent wrapper with tools
+    - Use Agent wrapper with tools
     - See client_with_agent.py for the hybrid pattern
-    - ChatAgent middleware intercepts and executes client tools locally
+    - Agent middleware intercepts and executes client tools locally
     - Server can have its own tools that execute server-side
     - Both client and server tools work together in same conversation
 
@@ -186,7 +186,7 @@ async def conversation_example(client: AGUIChatClient):
 
     # Check if context was maintained
     if "alice" not in response2.text.lower():
-        print("\n[Note: Server may not maintain thread context - consider using ChatAgent for history management]")
+        print("\n[Note: Server may not maintain thread context - consider using Agent for history management]")
 
     # Third turn
     print("\nUser: Can you also tell me what 10 * 5 is?\n")