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/packages/devui/tests/devui/test_discovery.py

@@ -74,7 +74,7 @@ async def test_discovery_accepts_agents_with_only_run():
 
         init_file = agent_dir / "__init__.py"
         init_file.write_text("""
-from agent_framework import AgentResponse, AgentThread, ChatMessage, Role, Content
+from agent_framework import AgentResponse, AgentThread, Message, Role, Content
 
 class NonStreamingAgent:
     id = "non_streaming"
@@ -83,7 +83,7 @@ class NonStreamingAgent:
 
     async def run(self, messages=None, *, thread=None, **kwargs):
         return AgentResponse(
-            messages=[ChatMessage(
+            messages=[Message(
                 role="assistant",
                 contents=[Content.from_text(text="response")]
             )],
@@ -188,14 +188,14 @@ workflow = WorkflowBuilder(start_executor=executor).build()
         agent_dir = temp_path / "my_agent"
         agent_dir.mkdir()
         (agent_dir / "agent.py").write_text("""
-from agent_framework import AgentResponse, AgentThread, ChatMessage, Role, TextContent
+from agent_framework import AgentResponse, AgentThread, Message, Role, TextContent
 
 class TestAgent:
     name = "Test Agent"
 
     async def run(self, messages=None, *, thread=None, **kwargs):
         return AgentResponse(
-            messages=[ChatMessage(role="assistant", contents=[Content.from_text(text="test")])],
+            messages=[Message(role="assistant", contents=[Content.from_text(text="test")])],
             response_id="test"
         )