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/agents/custom/custom_agent.py

@@ -9,8 +9,8 @@ from agent_framework import (
     AgentResponseUpdate,
     AgentThread,
     BaseAgent,
-    ChatMessage,
     Content,
+    Message,
     Role,
     normalize_messages,
 )
@@ -57,7 +57,7 @@ class EchoAgent(BaseAgent):
 
     def run(
         self,
-        messages: str | ChatMessage | list[str] | list[ChatMessage] | None = None,
+        messages: str | Message | list[str] | list[Message] | None = None,
         *,
         stream: bool = False,
         thread: AgentThread | None = None,
@@ -81,7 +81,7 @@ class EchoAgent(BaseAgent):
 
     async def _run(
         self,
-        messages: str | ChatMessage | list[str] | list[ChatMessage] | None = None,
+        messages: str | Message | list[str] | list[Message] | None = None,
         *,
         thread: AgentThread | None = None,
         **kwargs: Any,
@@ -91,11 +91,9 @@ class EchoAgent(BaseAgent):
         normalized_messages = normalize_messages(messages)
 
         if not normalized_messages:
-            response_message = ChatMessage(
+            response_message = Message(
                 role=Role.ASSISTANT,
-                contents=[
-                    Content.from_text(text="Hello! I'm a custom echo agent. Send me a message and I'll echo it back.")
-                ],
+                contents=[Content.from_text(text="Hello! I'm a custom echo agent. Send me a message and I'll echo it back.")],
             )
         else:
             # For simplicity, echo the last user message
@@ -105,7 +103,7 @@ class EchoAgent(BaseAgent):
             else:
                 echo_text = f"{self.echo_prefix}[Non-text message received]"
 
-            response_message = ChatMessage(role=Role.ASSISTANT, contents=[Content.from_text(text=echo_text)])
+            response_message = Message(role=Role.ASSISTANT, contents=[Content.from_text(text=echo_text)])
 
         # Notify the thread of new messages if provided
         if thread is not None:
@@ -115,7 +113,7 @@ class EchoAgent(BaseAgent):
 
     async def _run_stream(
         self,
-        messages: str | ChatMessage | list[str] | list[ChatMessage] | None = None,
+        messages: str | Message | list[str] | list[Message] | None = None,
         *,
         thread: AgentThread | None = None,
         **kwargs: Any,
@@ -150,7 +148,7 @@ class EchoAgent(BaseAgent):
 
         # Notify the thread of the complete response if provided
         if thread is not None:
-            complete_response = ChatMessage(role=Role.ASSISTANT, contents=[Content.from_text(text=response_text)])
+            complete_response = Message(role=Role.ASSISTANT, contents=[Content.from_text(text=response_text)])
             await self._notify_thread_of_new_messages(thread, normalized_messages, complete_response)