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/getting_started/devui/workflow_agents/workflow.py

@@ -59,10 +59,10 @@ def is_approved(message: Any) -> bool:
 
 
 # Create Azure OpenAI chat client
-chat_client = AzureOpenAIChatClient(api_key=os.environ.get("AZURE_OPENAI_API_KEY", ""))
+client = AzureOpenAIChatClient(api_key=os.environ.get("AZURE_OPENAI_API_KEY", ""))
 
 # Create Writer agent - generates content
-writer = chat_client.as_agent(
+writer = client.as_agent(
     name="Writer",
     instructions=(
         "You are an excellent content writer. "
@@ -72,7 +72,7 @@ writer = chat_client.as_agent(
 )
 
 # Create Reviewer agent - evaluates and provides structured feedback
-reviewer = chat_client.as_agent(
+reviewer = client.as_agent(
     name="Reviewer",
     instructions=(
         "You are an expert content reviewer. "
@@ -90,7 +90,7 @@ reviewer = chat_client.as_agent(
 )
 
 # Create Editor agent - improves content based on feedback
-editor = chat_client.as_agent(
+editor = client.as_agent(
     name="Editor",
     instructions=(
         "You are a skilled editor. "
@@ -101,7 +101,7 @@ editor = chat_client.as_agent(
 )
 
 # Create Publisher agent - formats content for publication
-publisher = chat_client.as_agent(
+publisher = client.as_agent(
     name="Publisher",
     instructions=(
         "You are a publishing agent. "
@@ -111,7 +111,7 @@ publisher = chat_client.as_agent(
 )
 
 # Create Summarizer agent - creates final publication report
-summarizer = chat_client.as_agent(
+summarizer = client.as_agent(
     name="Summarizer",
     instructions=(
         "You are a summarizer agent. "