feat(cli): expose session rewind on Agent Engine and api_server - #6704

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Link to Issue or Description of Change

• Closes: Feature Request: Expose rewind functionality via Agent Engine REST API #4121

Summary

Expose session rewind for Agent Engine / adk api_server deployments so edit-message / undo flows work without relying on the Vertex Sessions appendEvent proto (which rejects rewind_before_invocation_id).

What changed

• Register async_rewind in Agent Engine _AGENT_ENGINE_CLASS_METHODS (deploy metadata + allowlist)
• Bind an ADK-owned async_rewind on the Agent Engine AdkApp instance used by /api/reasoning_engine (calls runner.rewind_async, returns updated session)
• Add REST POST /apps/{app}/users/{user}/sessions/{session}/rewind

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

uv run pytest tests/unittests/cli/test_fast_api.py::test_rewind_session_endpoint \
  tests/unittests/cli/test_fast_api.py::test_rewind_session_not_found \
  tests/unittests/cli/test_fast_api.py::test_gemini_reasoning_engine_async_rewind \
  tests/unittests/cli/test_fast_api.py::test_async_rewind_is_registered_agent_engine_class_method -q
....                                                                     [100%]
4 passed

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

Additional context

cc @wuliang229 @yeesian — ready for review when you have a chance.