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/packages/lab/lightning/tests/test_lightning.py

@@ -9,7 +9,7 @@ import pytest
 
 agentlightning = pytest.importorskip("agentlightning")
 
-from agent_framework import AgentExecutor, AgentResponse, ChatAgent, WorkflowBuilder, Workflow
+from agent_framework import AgentExecutor, AgentResponse, Agent, WorkflowBuilder, Workflow
 from agent_framework_lab_lightning import AgentFrameworkTracer
 from agent_framework.openai import OpenAIChatClient
 from agentlightning import TracerTraceToTriplet
@@ -80,14 +80,14 @@ def workflow_two_agents():
             ),
         ):
             # Create the two agents
-            analyzer_agent = ChatAgent(
-                chat_client=first_chat_client,
+            analyzer_agent = Agent(
+                client=first_chat_client,
                 name="DataAnalyzer",
                 instructions="You are a data analyst. Analyze the given data and provide insights.",
             )
 
-            advisor_agent = ChatAgent(
-                chat_client=second_chat_client,
+            advisor_agent = Agent(
+                client=second_chat_client,
                 name="InvestmentAdvisor",
                 instructions="You are an investment advisor. Based on analysis results, provide recommendations.",
             )