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/CHANGELOG.md

@@ -10,6 +10,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 - **agent-framework-core**: Add long-running agents and background responses support with `ContinuationToken` TypedDict, `background` option in `OpenAIResponsesOptions`, and continuation token propagation through response types ([#2478](https://github.com/microsoft/agent-framework/issues/2478))
+### Changed
+
+- **agent-framework-core**: [BREAKING] Renamed core types for simpler API:
+  - `ChatAgent` → `Agent`
+  - `RawChatAgent` → `RawAgent`
+  - `ChatMessage` → `Message`
+  - `ChatClientProtocol` → `SupportsChatGetResponse`
 
 ## [1.0.0b260130] - 2026-01-30
 
@@ -272,7 +279,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - **agent-framework-core**: [BREAKING] Support Magentic agent tool call approvals and plan stalling HITL behavior (#2569)
-- **agent-framework-core**: [BREAKING] Standardize orchestration outputs as list of `ChatMessage`; allow agent as group chat manager (#2291)
+- **agent-framework-core**: [BREAKING] Standardize orchestration outputs as list of `Message`; allow agent as group chat manager (#2291)
 - **agent-framework-core**: [BREAKING] Respond with `AgentRunResponse` including serialized structured output (#2285)
 - **observability**: Use `executor_id` and `edge_group_id` as span names for clearer traces (#2538)
 - **agent-framework-devui**: Add multimodal input support for workflows and refactor chat input (#2593)
@@ -318,7 +325,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **agent-framework-core**: Fix tool execution bleed-over in aiohttp/Bot Framework scenarios ([#2314](https://github.com/microsoft/agent-framework/pull/2314))
 - **agent-framework-core**: `@ai_function` now correctly handles `self` parameter ([#2266](https://github.com/microsoft/agent-framework/pull/2266))
 - **agent-framework-core**: Resolve string annotations in `FunctionExecutor` ([#2308](https://github.com/microsoft/agent-framework/pull/2308))
-- **agent-framework-core**: Langfuse observability captures ChatAgent system instructions ([#2316](https://github.com/microsoft/agent-framework/pull/2316))
+- **agent-framework-core**: Langfuse observability captures Agent system instructions ([#2316](https://github.com/microsoft/agent-framework/pull/2316))
 - **agent-framework-core**: Incomplete URL substring sanitization fix ([#2274](https://github.com/microsoft/agent-framework/pull/2274))
 - **observability**: Handle datetime serialization in tool results ([#2248](https://github.com/microsoft/agent-framework/pull/2248))