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/samples/demos/chatkit-integration/README.md

@@ -38,7 +38,7 @@ graph TB
         subgraph Integration["Agent Framework Integration"]
             Converter[ThreadItemConverter]
             Streamer[stream_agent_response]
-            Agent[ChatAgent]
+            Agent[Agent]
         end
 
         Widgets[Widget Rendering<br/>render_weather_widget<br/>render_city_selector_widget]
@@ -61,7 +61,7 @@ graph TB
     AttStore -.->|save files| Files
     AttStore -.->|save metadata| SQLite
 
-    Converter -->|ChatMessage array| Agent
+    Converter -->|Message array| Agent
     Agent -->|AgentResponseUpdate| Streamer
     Streamer -->|ThreadStreamEvent| ChatKit
 
@@ -88,7 +88,7 @@ The sample implements a ChatKit server using the `ChatKitServer` base class from
 - **`WeatherChatKitServer`**: Custom ChatKit server implementation that:
 
   - Extends `ChatKitServer[dict[str, Any]]`
-  - Uses Agent Framework's `ChatAgent` with Azure OpenAI
+  - Uses Agent Framework's `Agent` with Azure OpenAI
   - Converts ChatKit messages to Agent Framework format using `ThreadItemConverter`
   - Streams responses back to ChatKit using `stream_agent_response`
   - Creates and streams interactive widgets after agent responses