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/ag-ui/README.md

@@ -14,15 +14,15 @@ pip install agent-framework-ag-ui
 
 ```python
 from fastapi import FastAPI
-from agent_framework import ChatAgent
+from agent_framework import Agent
 from agent_framework.azure import AzureOpenAIChatClient
 from agent_framework.ag_ui import add_agent_framework_fastapi_endpoint
 
 # Create your agent
-agent = ChatAgent(
+agent = Agent(
     name="my_agent",
     instructions="You are a helpful assistant.",
-    chat_client=AzureOpenAIChatClient(
+    client=AzureOpenAIChatClient(
         endpoint="https://your-resource.openai.azure.com/",
         deployment_name="gpt-4o-mini",
         api_key="your-api-key",
@@ -58,7 +58,7 @@ The `AGUIChatClient` supports:
 - Streaming and non-streaming responses
 - Hybrid tool execution (client-side + server-side tools)
 - Automatic thread management for conversation continuity
-- Integration with `ChatAgent` for client-side history management
+- Integration with `Agent` for client-side history management
 
 ## Documentation
 
@@ -91,7 +91,7 @@ The AG-UI endpoint does not enforce authentication by default. **For production
 import os
 from fastapi import Depends, FastAPI, HTTPException, Security
 from fastapi.security import APIKeyHeader
-from agent_framework import ChatAgent
+from agent_framework import Agent
 from agent_framework.ag_ui import add_agent_framework_fastapi_endpoint
 
 # Configure API key authentication
@@ -104,7 +104,7 @@ async def verify_api_key(api_key: str | None = Security(API_KEY_HEADER)) -> None
         raise HTTPException(status_code=401, detail="Invalid or missing API key")
 
 # Create agent and app
-agent = ChatAgent(name="my_agent", instructions="...", chat_client=...)
+agent = Agent(name="my_agent", instructions="...", client=...)
 app = FastAPI()
 
 # Register endpoint WITH authentication