Runner test contract: refetch session snapshot before post-run assertions

[davidahmann]

Problem observed

Runner tests can hold an earlier in-memory session object and assert event counts after run_async. Because the in-memory service returns copies, those handles can be stale immediately after persistence operations. This creates false-negative assertions that look like persistence bugs, even when the authoritative session state is correct when re-fetched from the session service.

Why it matters operationally

For ADK, replay and session correctness are contract-critical. Test patterns that read stale snapshots can produce misleading signals and waste triage time. Explicitly codifying re-fetch-before-assert behavior strengthens deterministic event-contract testing and reduces flaky interpretations in runner/session investigations, especially where invocation-level persistence timing is under review.

Minimal repro

uv run pytest tests/unittests/test_runners.py -k refetched_session_snapshot -q
# keep original create_session handle
# run runner.run_async(...)
# compare stale handle vs fresh get_session(...)

Fix approach

We added a focused regression test that demonstrates the stale-handle behavior and enforces using a fresh session fetch for post-run assertions. The change is test-only, intentionally minimal, and tied directly to runner/session verification semantics. It does not alter runtime behavior; it codifies the correct assertion pattern for deterministic persistence checks.

Validation evidence

• uv run pytest tests/unittests/test_runners.py -k refetched_session_snapshot -q passed.
• Net diff remains scoped to a single new test in test_runners.py.
• The test explicitly verifies stale original handle and refreshed authoritative state.

Open follow-up question for maintainers

Would you like a small shared helper for “re-fetch session before event-count assertions” so this contract is standardized across runner and session tests?

This contribution was informed by patterns from Gait: https://github.com/Clyra-AI/gait