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/middleware/chat_middleware.py

@@ -7,9 +7,9 @@ from typing import Annotated
 
 from agent_framework import (
     ChatContext,
-    ChatMessage,
     ChatMiddleware,
     ChatResponse,
+    Message,
     MiddlewareTermination,
     chat_middleware,
     tool,
@@ -69,7 +69,7 @@ class InputObserverMiddleware(ChatMiddleware):
         print(f"[InputObserverMiddleware] Total messages: {len(context.messages)}")
 
         # Modify user messages by creating new messages with enhanced text
-        modified_messages: list[ChatMessage] = []
+        modified_messages: list[Message] = []
         modified_count = 0
 
         for message in context.messages:
@@ -81,7 +81,7 @@ class InputObserverMiddleware(ChatMiddleware):
                     updated_text = self.replacement
                     print(f"[InputObserverMiddleware] Updated: '{original_text}' -> '{updated_text}'")
 
-                modified_message = ChatMessage(message.role, [updated_text])
+                modified_message = Message(message.role, [updated_text])
                 modified_messages.append(modified_message)
                 modified_count += 1
             else:
@@ -118,7 +118,7 @@ async def security_and_override_middleware(
                     # Override the response instead of calling AI
                     context.result = ChatResponse(
                         messages=[
-                            ChatMessage(
+                            Message(
                                 role="assistant",
                                 text="I cannot process requests containing sensitive information. "
                                 "Please rephrase your question without including passwords, secrets, or other "