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/core/AGENTS.md

@@ -9,7 +9,7 @@ agent_framework/
 ├── __init__.py          # Public API exports
 ├── _agents.py           # Agent implementations
 ├── _clients.py          # Chat client base classes and protocols
-├── _types.py            # Core types (ChatMessage, ChatResponse, Content, etc.)
+├── _types.py            # Core types (Message, ChatResponse, Content, etc.)
 ├── _tools.py            # Tool definitions and function invocation
 ├── _middleware.py       # Middleware system for request/response interception
 ├── _threads.py          # AgentThread and message store abstractions
@@ -27,16 +27,16 @@ agent_framework/
 
 - **`SupportsAgentRun`** - Protocol defining the agent interface
 - **`BaseAgent`** - Abstract base class for agents
-- **`ChatAgent`** - Main agent class wrapping a chat client with tools, instructions, and middleware
+- **`Agent`** - Main agent class wrapping a chat client with tools, instructions, and middleware
 
 ### Chat Clients (`_clients.py`)
 
-- **`ChatClientProtocol`** - Protocol for chat client implementations
+- **`SupportsChatGetResponse`** - Protocol for chat client implementations
 - **`BaseChatClient`** - Abstract base class with middleware support; subclasses implement `_inner_get_response()` and `_inner_get_streaming_response()`
 
 ### Types (`_types.py`)
 
-- **`ChatMessage`** - Represents a chat message with role, content, and metadata
+- **`Message`** - Represents a chat message with role, content, and metadata
 - **`ChatResponse`** - Response from a chat client containing messages and usage
 - **`ChatResponseUpdate`** - Streaming response update
 - **`AgentResponse`** / **`AgentResponseUpdate`** - Agent-level response wrappers
@@ -91,11 +91,11 @@ agent_framework/
 ### Creating an Agent
 
 ```python
-from agent_framework import ChatAgent
+from agent_framework import Agent
 from agent_framework.openai import OpenAIChatClient
 
-agent = ChatAgent(
-    chat_client=OpenAIChatClient(),
+agent = Agent(
+    client=OpenAIChatClient(),
     instructions="You are helpful.",
     tools=[my_function],
 )
@@ -114,7 +114,7 @@ agent = OpenAIChatClient().as_agent(
 ### Middleware Pipeline
 
 ```python
-from agent_framework import ChatAgent, AgentMiddleware, AgentContext
+from agent_framework import Agent, AgentMiddleware, AgentContext
 
 class LoggingMiddleware(AgentMiddleware):
     async def process(self, context: AgentContext, call_next) -> None:
@@ -122,18 +122,18 @@ class LoggingMiddleware(AgentMiddleware):
         await call_next(context)
         print(f"Output: {context.result}")
 
-agent = ChatAgent(..., middleware=[LoggingMiddleware()])
+agent = Agent(..., middleware=[LoggingMiddleware()])
 ```
 
 ### Custom Chat Client
 
 ```python
-from agent_framework import BaseChatClient, ChatResponse, ChatMessage
+from agent_framework import BaseChatClient, ChatResponse, Message
 
 class MyClient(BaseChatClient):
     async def _inner_get_response(self, *, messages, options, **kwargs) -> ChatResponse:
         # Call your LLM here
-        return ChatResponse(messages=[ChatMessage(role="assistant", text="Hi!")])
+        return ChatResponse(messages=[Message(role="assistant", text="Hi!")])
 
     async def _inner_get_streaming_response(self, *, messages, options, **kwargs):
         yield ChatResponseUpdate(...)