feat(models): add get_update_marker() for google-adk 1.28.0 compatibility

[Alberto-Codes]

google-adk 1.28.0 introduces get_update_marker() on StorageSession for string-based optimistic concurrency, replacing float timestamp comparison. Without this method, EncryptedStorageSession raises AttributeError when append_event() is called under 1.28.0.

• Add get_update_marker() returning ISO 8601 with microsecond precision
• Add update_timestamp_tz property (closes pre-existing parity gap with upstream)
• Update to_session() to stamp _storage_update_marker on returned Session
• Add sentinel test verifying duck-type parity with ADK StorageSession
• Add unit tests for marker format, UTC normalization, and round-trip

Test: uv run pytest tests/unit/test_models.py tests/integration/test_adk_conformance.py -v

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PR Review

Checklist

• Self-reviewed my code
• Tests pass (uv run pytest)
• Lint passes (uv run ruff check .)
• Breaking changes use ! in title and BREAKING CHANGE: in body

Review Focus

• update_timestamp_tz hardcodes is_sqlite=False (diverges from upstream's dynamic detection via inspect(self).session). Documented in a Note: block. Revisit after 1.28.0 upgrade.
• _storage_update_marker is set via direct assignment on Session (valid Pydantic v2 underscore attr). In 1.28.0 it becomes a proper PrivateAttr(default=None).

Related

• Upstream commit: google/adk-python@b8e7647 (1.28.0)
• Upstream issue: DatabaseSessionService.append_event reload-and-continue in 1.24+ corrupts session under concurrent run_async google/adk-python#4751

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[Copilot]

Pull request overview

Adds forward-compatibility with google-adk 1.28.0’s string-based optimistic concurrency by implementing get_update_marker()/update_timestamp_tz parity on the encrypted StorageSession model and ensuring returned Session objects are stamped with _storage_update_marker.

Changes:

• Add EncryptedStorageSession.get_update_marker() and update_timestamp_tz for ADK StorageSession parity.
• Update to_session() to stamp _storage_update_marker on the returned ADK Session.
• Add unit + integration sentinel tests to validate marker behavior and duck-typed parity with ADK’s StorageSession.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/adk_secure_sessions/services/models.py	Implements get_update_marker(), adds update_timestamp_tz, and stamps _storage_update_marker in to_session() for optimistic concurrency compatibility.
tests/unit/test_models.py	Adds unit tests for marker formatting/UTC normalization, update_timestamp_tz, and _storage_update_marker stamping.
tests/integration/test_adk_conformance.py	Adds a sentinel parity test against upstream StorageSession and round-trip assertions that _storage_update_marker is populated.
_bmad-output/implementation-artifacts/tech-spec-adk-128-get-update-marker.md	Documents the implementation plan and acceptance criteria for the ADK 1.28.0 compatibility work.

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