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/README.md

@@ -62,7 +62,7 @@ You can also override environment variables by explicitly passing configuration
 ```python
 from agent_framework.azure import AzureOpenAIChatClient
 
-chat_client = AzureOpenAIChatClient(
+client = AzureOpenAIChatClient(
     api_key='',
     endpoint='',
     deployment_name='',
@@ -78,12 +78,12 @@ Create agents and invoke them directly:
 
 ```python
 import asyncio
-from agent_framework import ChatAgent
+from agent_framework import Agent
 from agent_framework.openai import OpenAIChatClient
 
 async def main():
-    agent = ChatAgent(
-        chat_client=OpenAIChatClient(),
+    agent = Agent(
+        client=OpenAIChatClient(),
         instructions="""
         1) A robot may not injure a human being...
         2) A robot must obey orders given it by human beings...
@@ -106,15 +106,15 @@ You can use the chat client classes directly for advanced workflows:
 
 ```python
 import asyncio
-from agent_framework import ChatMessage
+from agent_framework import Message
 from agent_framework.openai import OpenAIChatClient
 
 async def main():
     client = OpenAIChatClient()
 
     messages = [
-        ChatMessage("system", ["You are a helpful assistant."]),
-        ChatMessage("user", ["Write a haiku about Agent Framework."])
+        Message("system", ["You are a helpful assistant."]),
+        Message("user", ["Write a haiku about Agent Framework."])
     ]
 
     response = await client.get_response(messages)
@@ -140,7 +140,7 @@ import asyncio
 from typing import Annotated
 from random import randint
 from pydantic import Field
-from agent_framework import ChatAgent
+from agent_framework import Agent
 from agent_framework.openai import OpenAIChatClient
 
 
@@ -162,8 +162,8 @@ def get_menu_specials() -> str:
 
 
 async def main():
-    agent = ChatAgent(
-        chat_client=OpenAIChatClient(),
+    agent = Agent(
+        client=OpenAIChatClient(),
         instructions="You are a helpful assistant that can provide weather and restaurant information.",
         tools=[get_weather, get_menu_specials]
     )
@@ -189,20 +189,20 @@ Coordinate multiple agents to collaborate on complex tasks using orchestration p
 
 ```python
 import asyncio
-from agent_framework import ChatAgent
+from agent_framework import Agent
 from agent_framework.openai import OpenAIChatClient
 
 
 async def main():
     # Create specialized agents
-    writer = ChatAgent(
-        chat_client=OpenAIChatClient(),
+    writer = Agent(
+        client=OpenAIChatClient(),
         name="Writer",
         instructions="You are a creative content writer. Generate and refine slogans based on feedback."
     )
 
-    reviewer = ChatAgent(
-        chat_client=OpenAIChatClient(),
+    reviewer = Agent(
+        client=OpenAIChatClient(),
         name="Reviewer",
         instructions="You are a critical reviewer. Provide detailed feedback on proposed slogans."
     )