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/threads/custom_chat_message_store_thread.py

@@ -4,7 +4,7 @@ import asyncio
 from collections.abc import Collection
 from typing import Any
 
-from agent_framework import ChatMessage, ChatMessageStoreProtocol
+from agent_framework import ChatMessageStoreProtocol, Message
 from agent_framework._threads import ChatMessageStoreState
 from agent_framework.openai import OpenAIChatClient
 
@@ -21,15 +21,15 @@ class CustomChatMessageStore(ChatMessageStoreProtocol):
     """Implementation of custom chat message store.
     In real applications, this can be an implementation of relational database or vector store."""
 
-    def __init__(self, messages: Collection[ChatMessage] | None = None) -> None:
-        self._messages: list[ChatMessage] = []
+    def __init__(self, messages: Collection[Message] | None = None) -> None:
+        self._messages: list[Message] = []
         if messages:
             self._messages.extend(messages)
 
-    async def add_messages(self, messages: Collection[ChatMessage]) -> None:
+    async def add_messages(self, messages: Collection[Message]) -> None:
         self._messages.extend(messages)
 
-    async def list_messages(self) -> list[ChatMessage]:
+    async def list_messages(self) -> list[Message]:
         return self._messages
 
     @classmethod