fix(sessions): update session last update time on vertex append event

[AbQaadir]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: Clarify Vertex append_event Session.last_update_time after append #7338

Problem:
In VertexAiSessionService.append_event, session.last_update_time was not updated when an event was appended to a session, unlike InMemorySessionService, SqliteSessionService, and DatabaseSessionService which keep session.last_update_time synchronized with event.timestamp. As a result, the in-memory Session object retained its initial creation time after events were added, drifting from the remote API state (which updates update_time) and other session implementations.

Solution:
In VertexAiSessionService.append_event, set session.last_update_time = event.timestamp when committing non-partial events to the session. Also added comprehensive unit tests covering:

• Verifying session.last_update_time advances on non-partial append and matches the retrieved remote session.
• Verifying multiple sequential appends continuously advance session.last_update_time.
• Verifying partial/streaming events (partial=True) do not update session.last_update_time or commit to session.events.

Testing Plan

Unit Tests:

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

Summary of passed tests:

• tests/unittests/sessions/test_vertex_ai_session_service.py: 57 passed
• tests/unittests/sessions/: 523 passed
• tests/unittests/events/ tests/unittests/runners/ tests/unittests/agents/: 1094 passed
• Static pre-commit checks (pyink, ruff, isort, addlicense, compliance checks): all passed
• mypy: 0 issues found in source

Manual End-to-End (E2E) Tests:

Run pytest with uv:

uv run --all-extras pytest tests/unittests/sessions/test_vertex_ai_session_service.py -k "test_append_event"

Verify all append event tests (test_append_event, test_append_event_updates_session_last_update_time, test_append_event_partial_does_not_update_session_last_update_time, test_append_multiple_events_advances_last_update_time) pass.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• 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.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

[adk-bot]

Thank you @AbQaadir for your contribution! 🎉

Your changes have been successfully imported and merged via Copybara in commit 11d92d7.

Closing this PR as the changes are now in the main branch.