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/devui/azure_responses_agent/agent.py

@@ -21,7 +21,7 @@ import logging
 import os
 from typing import Annotated
 
-from agent_framework import ChatAgent, tool
+from agent_framework import Agent, tool
 from agent_framework.azure import AzureOpenAIResponsesClient
 
 logger = logging.getLogger(__name__)
@@ -68,7 +68,7 @@ def extract_key_points(
 
 
 # Agent using Azure OpenAI Responses API (supports PDF uploads!)
-agent = ChatAgent(
+agent = Agent(
     name="AzureResponsesAgent",
     description="An agent that can analyze PDFs, images, and other documents using Azure OpenAI Responses API",
     instructions="""
@@ -85,7 +85,7 @@ agent = ChatAgent(
     For PDFs, you can read and understand the text, tables, and structure.
     For images, you can describe what you see and extract any text.
     """,
-    chat_client=AzureOpenAIResponsesClient(
+    client=AzureOpenAIResponsesClient(
         deployment_name=_deployment_name,
         endpoint=_endpoint,
         api_version="2025-03-01-preview",  # Required for Responses API