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/semantic-kernel-migration/chat_completion/01_basic_chat_completion.py

@@ -8,7 +8,7 @@
 #   uv run samples/semantic-kernel-migration/chat_completion/01_basic_chat_completion.py
 
 # Copyright (c) Microsoft. All rights reserved.
-"""Basic SK ChatCompletionAgent vs Agent Framework ChatAgent.
+"""Basic SK ChatCompletionAgent vs Agent Framework Agent.
 
 Both samples expect OpenAI-compatible environment variables (OPENAI_API_KEY or
 Azure OpenAI configuration). Update the prompts or client wiring to match your
@@ -34,10 +34,10 @@ async def run_semantic_kernel() -> None:
 
 
 async def run_agent_framework() -> None:
-    """Call Agent Framework's ChatAgent created from OpenAIChatClient."""
+    """Call Agent Framework's Agent created from OpenAIChatClient."""
     from agent_framework.openai import OpenAIChatClient
 
-    # AF constructs a lightweight ChatAgent backed by OpenAIChatClient.
+    # AF constructs a lightweight Agent backed by OpenAIChatClient.
     chat_agent = OpenAIChatClient().as_agent(
         name="Support",
         instructions="Answer in one sentence.",