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/orchestrations/magentic_checkpoint.py

@@ -6,9 +6,9 @@ from pathlib import Path
 from typing import cast
 
 from agent_framework import (
-    ChatAgent,
-    ChatMessage,
+    Agent,
     FileCheckpointStorage,
+    Message,
     WorkflowCheckpoint,
     WorkflowEvent,
     WorkflowRunState,
@@ -52,26 +52,26 @@ def build_workflow(checkpoint_storage: FileCheckpointStorage):
 
     # Two vanilla ChatAgents act as participants in the orchestration. They do not need
     # extra state handling because their inputs/outputs are fully described by chat messages.
-    researcher = ChatAgent(
+    researcher = Agent(
         name="ResearcherAgent",
         description="Collects background facts and references for the project.",
         instructions=("You are the research lead. Gather crisp bullet points the team should know."),
-        chat_client=AzureOpenAIChatClient(credential=AzureCliCredential()),
+        client=AzureOpenAIChatClient(credential=AzureCliCredential()),
     )
 
-    writer = ChatAgent(
+    writer = Agent(
         name="WriterAgent",
         description="Synthesizes the final brief for stakeholders.",
         instructions=("You convert the research notes into a structured brief with milestones and risks."),
-        chat_client=AzureOpenAIChatClient(credential=AzureCliCredential()),
+        client=AzureOpenAIChatClient(credential=AzureCliCredential()),
     )
 
     # Create a manager agent for orchestration
-    manager_agent = ChatAgent(
+    manager_agent = Agent(
         name="MagenticManager",
         description="Orchestrator that coordinates the research and writing workflow",
         instructions="You coordinate a team to complete complex tasks efficiently.",
-        chat_client=AzureOpenAIChatClient(credential=AzureCliCredential()),
+        client=AzureOpenAIChatClient(credential=AzureCliCredential()),
     )
 
     # The builder wires in the Magentic orchestrator, sets the plan review path, and
@@ -167,7 +167,7 @@ async def main() -> None:
     if not result:
         print("No result data from workflow.")
         return
-    output_messages = cast(list[ChatMessage], result)
+    output_messages = cast(list[Message], result)
     print("\n=== Final Answer ===")
     # The output of the Magentic workflow is a list of ChatMessages with only one final message
     # generated by the orchestrator.
@@ -234,7 +234,7 @@ async def main() -> None:
         print("No result data from post-plan resume.")
         return
 
-    output_messages = cast(list[ChatMessage], post_result)
+    output_messages = cast(list[Message], post_result)
     print("\n=== Final Answer (post-plan resume) ===")
     # The output of the Magentic workflow is a list of ChatMessages with only one final message
     # generated by the orchestrator.