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/samples/semantic-kernel-migration/orchestrations/group_chat.py

@@ -16,7 +16,7 @@ import sys
 from collections.abc import Sequence
 from typing import Any, cast
 
-from agent_framework import ChatAgent, ChatMessage
+from agent_framework import Agent, Message
 from agent_framework.azure import AzureOpenAIChatClient, AzureOpenAIResponsesClient
 from agent_framework.orchestrations import GroupChatBuilder
 from azure.identity import AzureCliCredential
@@ -224,21 +224,21 @@ async def run_semantic_kernel_example(task: str) -> str:
 async def run_agent_framework_example(task: str) -> str:
     credential = AzureCliCredential()
 
-    researcher = ChatAgent(
+    researcher = Agent(
         name="Researcher",
         description="Collects background information and potential resources.",
         instructions=(
             "Gather concise facts or considerations that help plan a community hackathon. "
             "Keep your responses factual and scannable."
         ),
-        chat_client=AzureOpenAIChatClient(credential=credential),
+        client=AzureOpenAIChatClient(credential=credential),
     )
 
-    planner = ChatAgent(
+    planner = Agent(
         name="Planner",
         description="Turns the collected notes into a concrete action plan.",
         instructions=("Propose a structured action plan that accounts for logistics, roles, and timeline."),
-        chat_client=AzureOpenAIResponsesClient(credential=credential),
+        client=AzureOpenAIResponsesClient(credential=credential),
     )
 
     workflow = GroupChatBuilder(
@@ -253,7 +253,7 @@ async def run_agent_framework_example(task: str) -> str:
             if isinstance(data, list) and len(data) > 0:
                 # Get the final message from the conversation
                 final_message = data[-1]
-                final_response = final_message.text or "" if isinstance(final_message, ChatMessage) else str(data)
+                final_response = final_message.text or "" if isinstance(final_message, Message) else str(data)
             else:
                 final_response = str(data)
     return final_response