fix: include rewind_before_invocation_id in VertexAiSessionService.append_event

[yuting0624]

Summary

Fixes #4930

VertexAiSessionService.append_event() was not including rewind_before_invocation_id in the actions dict sent to the Vertex AI Agent Engine API. This caused rewind events to lose their target invocation ID when persisted, making session rewind non-functional for Agent Engine deployments.

Changes

• src/google/adk/sessions/vertex_ai_session_service.py: Added rewind_before_invocation_id to the actions dict in append_event() (3 lines)
• tests/unittests/sessions/test_vertex_ai_session_service.py: Added test_append_event_with_rewind to verify the field round-trips correctly through append and get

Root Cause

The actions dict construction in append_event() explicitly maps each field from EventActions to the API payload, but rewind_before_invocation_id was omitted. Other fields like transfer_to_agent, state_delta, skip_summarization, etc. were all present.

[adk-bot]

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[lucasbarzotto-axonify]

Hi @yuting0624, does this fix works?

I tried it and got the following error:

Error rewinding session 137352779248173056: 1 validation error for _AppendAgentEngineSessionEventRequestParameters

config.actions.rewind_before_invocation_id

Extra inputs are not permitted [type=extra_forbidden, input_value='p-146156f2-8f8e-42ae-af2a-75d00a48130c', input_type=str]

For further information visit https://errors.pydantic.dev/2.12/v/extra_forbidden

Looking at this, it looks like the rewind_before_invocation_id field is not allowed?

Should we use custom_metadata instead? The same way it's done for event compaction?

    # Store compaction data in custom_metadata since the Vertex AI service
    # does not yet support the compaction field.
    # TODO: Stop writing to custom_metadata once the Vertex AI service
    # supports the compaction field natively in EventActions.
    if event.actions and event.actions.compaction:
      compaction_dict = event.actions.compaction.model_dump(
          exclude_none=True, mode='json'
      )
      _set_internal_custom_metadata(
          metadata_dict,
          key=_COMPACTION_CUSTOM_METADATA_KEY,
          value=compaction_dict,
      )

[yuting0624]

Thanks for catching this @lucasbarzotto-axonify! You're absolutely right — the Vertex AI SDK doesn't support rewind_before_invocation_id in EventActions yet.

I've pushed a fix that stores it in custom_metadata following the same pattern as compaction:

• Write path: rewind_before_invocation_id is now stored via _set_internal_custom_metadata() instead of directly in the actions dict
• Read path: Extracted from custom_metadata and restored into EventActions on retrieval

All 25 session service tests pass. The Pydantic validation error should no longer occur.