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/README.md

@@ -17,7 +17,7 @@ pip install agent-framework-devui --pre
 You can also launch it programmatically
 
 ```python
-from agent_framework import ChatAgent
+from agent_framework import Agent
 from agent_framework.openai import OpenAIChatClient
 from agent_framework.devui import serve
 
@@ -26,9 +26,9 @@ def get_weather(location: str) -> str:
     return f"Weather in {location}: 72°F and sunny"
 
 # Create your agent
-agent = ChatAgent(
+agent = Agent(
     name="WeatherAgent",
-    chat_client=OpenAIChatClient(),
+    client=OpenAIChatClient(),
     tools=[get_weather]
 )
 
@@ -55,8 +55,8 @@ When DevUI starts with no discovered entities, it displays a **sample entity gal
 
 ```python
 # ✅ Correct - DevUI handles cleanup automatically
-mcp_tool = MCPStreamableHTTPTool(url="http://localhost:8011/mcp", chat_client=chat_client)
-agent = ChatAgent(tools=mcp_tool)
+mcp_tool = MCPStreamableHTTPTool(url="http://localhost:8011/mcp", client=client)
+agent = Agent(tools=mcp_tool)
 serve(entities=[agent])
 ```
 
@@ -68,13 +68,13 @@ Register cleanup hooks to properly close credentials and resources on shutdown:
 
 ```python
 from azure.identity.aio import DefaultAzureCredential
-from agent_framework import ChatAgent
+from agent_framework import Agent
 from agent_framework.azure import AzureOpenAIChatClient
 from agent_framework_devui import register_cleanup, serve
 
 credential = DefaultAzureCredential()
 client = AzureOpenAIChatClient()
-agent = ChatAgent(name="MyAgent", chat_client=client)
+agent = Agent(name="MyAgent", client=client)
 
 # Register cleanup hook - credential will be closed on shutdown
 register_cleanup(agent, credential.close)
@@ -92,7 +92,7 @@ For your agents to be discovered by the DevUI, they must be organized in a direc
 ```
 agents/
 ├── weather_agent/
-│   ├── __init__.py      # Must export: agent = ChatAgent(...)
+│   ├── __init__.py      # Must export: agent = Agent(...)
 │   ├── agent.py
 │   └── .env             # Optional: API keys, config vars
 ├── my_workflow/