fix(firestore): preserve update timestamps in list_sessions

[kkj333]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):
Closes: #5632

Problem

FirestoreSessionService.list_sessions() always sets Session.last_update_time to 0.0, even when Firestore documents contain updateTime.

Solution

Use the same timestamp conversion logic as get_session() in list_sessions():

• if updateTime is a datetime, use .timestamp()
• otherwise, attempt float(updateTime)
• fallback to 0.0 when conversion fails
  This keeps list responses consistent with get_session() and preserves accurate update timestamps.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.
  Updated tests:
• test_list_sessions_with_user_id
• test_list_sessions_without_user_id
  Local pytest summary:
• tests/unittests/integrations/firestore/test_firestore_session_service.py
• 17 passed
  Manual End-to-End (E2E) Tests:
  Reproduced with a mocked Firestore client where session docs include updateTime; before fix last_update_time was 0.0, after fix it matches the Firestore value.

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.

[google-cla]

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[kkj333]

The failing check is caused by the triaging workflow configuration, not by this PR changes.

adk_pr_triaging_agent fails with:
404 NOT_FOUND: models/gemini-2.5-pro is not found for API version v1beta

Could a maintainer please update the triaging agent model (or rerun after fixing the workflow config)?