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/mcp/mcp_api_key_auth.py

@@ -2,7 +2,7 @@
 
 import os
 
-from agent_framework import ChatAgent, MCPStreamableHTTPTool
+from agent_framework import Agent, MCPStreamableHTTPTool
 from agent_framework.openai import OpenAIResponsesClient
 from httpx import AsyncClient
 
@@ -43,8 +43,8 @@ async def api_key_auth_example() -> None:
             url=mcp_server_url,
             http_client=http_client,  # Pass HTTP client with authentication headers
         ) as mcp_tool,
-        ChatAgent(
-            chat_client=OpenAIResponsesClient(),
+        Agent(
+            client=OpenAIResponsesClient(),
             name="Agent",
             instructions="You are a helpful assistant.",
             tools=mcp_tool,

python/samples/getting_started/mcp/mcp_github_pat.py

@@ -3,7 +3,7 @@
 import asyncio
 import os
 
-from agent_framework import ChatAgent, HostedMCPTool
+from agent_framework import Agent, HostedMCPTool
 from agent_framework.openai import OpenAIResponsesClient
 from dotenv import load_dotenv
 
@@ -54,8 +54,8 @@ async def github_mcp_example() -> None:
     )
 
     # 5. Create agent with the GitHub MCP tool
-    async with ChatAgent(
-        chat_client=OpenAIResponsesClient(),
+    async with Agent(
+        client=OpenAIResponsesClient(),
         name="GitHubAgent",
         instructions=(
             "You are a helpful assistant that can help users interact with GitHub. "