Enforce deterministic persisted-event counts after run_async

[davidahmann]

Runner/session tests can misreport event totals when assertions rely on stale session snapshots, reducing confidence in replay behavior.

Current behavior is insufficient because post-invocation assertions sometimes count in-memory state that is no longer authoritative for persisted session events.

Why now: session replay and event-contract stability are core runtime guarantees and this failure pattern is recurring.

Claim-to-codepath map:

• Invocation persistence flow in src/google/adk/runners.py.
• Session storage/read paths in src/google/adk/sessions.
• Assertions and fixtures in tests/unittests.

Requested behavior: tests re-fetch session state before counting events and demonstrate deterministic totals across repeated runs.

Evidence Packet

• Commit under test: aa77920
• Runtime environment:
  • OS: Darwin 25.3.0 arm64
  • Python 3.14.0
  • go version go1.25.7 darwin/arm64
  • Node: v22.19.0
• Minimal repro:
  1. Follow the repo's current local validation/CI-triage path.
  2. Observe the behavior described above for this issue.
• Expected behavior: deterministic, documented behavior matching acceptance criteria below.
• Actual behavior: behavior can be inconsistent for the scenario described above.

Acceptance Criteria

• Implement the smallest code change that addresses the proven pain point.
• Add focused regression test coverage for the changed behavior.
• Ensure backward compatibility expectations are explicit in tests/docs.

[llalitkumarrr]

Hello @davidahmann ,

Could you please elaborate on this issue to help understand this a bit better?