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/getting_started/workflows/_start-here/step2_agents_in_a_workflow.py

@@ -27,15 +27,15 @@ Prerequisites:
 async def main():
     """Build and run a simple two node agent workflow: Writer then Reviewer."""
     # Create the Azure chat client. AzureCliCredential uses your current az login.
-    chat_client = AzureOpenAIChatClient(credential=AzureCliCredential())
-    writer_agent = chat_client.as_agent(
+    client = AzureOpenAIChatClient(credential=AzureCliCredential())
+    writer_agent = client.as_agent(
         instructions=(
             "You are an excellent content writer. You create new content and edit contents based on the feedback."
         ),
         name="writer",
     )
 
-    reviewer_agent = chat_client.as_agent(
+    reviewer_agent = client.as_agent(
         instructions=(
             "You are an excellent content reviewer."
             "Provide actionable feedback to the writer about the provided content."

python/samples/getting_started/workflows/_start-here/step3_streaming.py

@@ -2,7 +2,7 @@
 
 import asyncio
 
-from agent_framework import AgentResponseUpdate, ChatMessage, WorkflowBuilder
+from agent_framework import AgentResponseUpdate, Message, WorkflowBuilder
 from agent_framework.azure import AzureOpenAIChatClient
 from azure.identity import AzureCliCredential
 
@@ -26,15 +26,15 @@ Prerequisites:
 async def main():
     """Build the two node workflow and run it with streaming to observe events."""
     # Create the Azure chat client. AzureCliCredential uses your current az login.
-    chat_client = AzureOpenAIChatClient(credential=AzureCliCredential())
-    writer_agent = chat_client.as_agent(
+    client = AzureOpenAIChatClient(credential=AzureCliCredential())
+    writer_agent = client.as_agent(
         instructions=(
             "You are an excellent content writer. You create new content and edit contents based on the feedback."
         ),
         name="writer",
     )
 
-    reviewer_agent = chat_client.as_agent(
+    reviewer_agent = client.as_agent(
         instructions=(
             "You are an excellent content reviewer."
             "Provide actionable feedback to the writer about the provided content."
@@ -52,7 +52,7 @@ async def main():
 
     # Run the workflow with the user's initial message and stream events as they occur.
     async for event in workflow.run(
-        ChatMessage("user", ["Create a slogan for a new electric SUV that is affordable and fun to drive."]),
+        Message("user", ["Create a slogan for a new electric SUV that is affordable and fun to drive."]),
         stream=True,
     ):
         # The outputs of the workflow are whatever the agents produce. So the events are expected to