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/CODING_STANDARD.md

@@ -118,10 +118,10 @@ Prefer attributes over inheritance when parameters are mostly the same:
 
 ```python
 # ✅ Preferred - using attributes
-from agent_framework import ChatMessage
+from agent_framework import Message
 
-user_msg = ChatMessage("user", ["Hello, world!"])
-asst_msg = ChatMessage("assistant", ["Hello, world!"])
+user_msg = Message("user", ["Hello, world!"])
+asst_msg = Message("assistant", ["Hello, world!"])
 
 # ❌ Not preferred - unnecessary inheritance
 from agent_framework import UserMessage, AssistantMessage
@@ -157,7 +157,7 @@ The package follows a flat import structure:
 
 - **Core**: Import directly from `agent_framework`
   ```python
-  from agent_framework import ChatAgent, tool
+  from agent_framework import Agent, tool
   ```
 
 - **Components**: Import from `agent_framework.<component>`
@@ -381,12 +381,12 @@ def create_client(
 Use Google-style docstrings for all public APIs:
 
 ```python
-def create_agent(name: str, chat_client: ChatClientProtocol) -> Agent:
+def create_agent(name: str, client: SupportsChatGetResponse) -> Agent:
     """Create a new agent with the specified configuration.
 
     Args:
         name: The name of the agent.
-        chat_client: The chat client to use for communication.
+        client: The chat client to use for communication.
 
     Returns:
         True if the strings are the same, False otherwise.
@@ -409,10 +409,10 @@ Define `__all__` in each module to explicitly declare the public API. Avoid usin
 
 ```python
 # ✅ Preferred - explicit __all__ and imports
-__all__ = ["ChatAgent", "ChatMessage", "ChatResponse"]
+__all__ = ["Agent", "Message", "ChatResponse"]
 
-from ._agents import ChatAgent
-from ._types import ChatMessage, ChatResponse
+from ._agents import Agent
+from ._types import Message, ChatResponse
 
 # ❌ Avoid - star imports
 from ._agents import *