Add test/unit/reliability_runner_test.dart with 3 fake_async stress tests
Verify exponential backoff caps at 900 s after repeated failures
Verify backoff resets to minimum after a successful sync
Verify concurrent multi-account failures all stay bounded
Test plan
task check-fast passes locally
All 3 new tests pass with fake_async time control
## Summary
- Add `test/unit/reliability_runner_test.dart` with 3 `fake_async` stress tests
- Verify exponential backoff caps at 900 s after repeated failures
- Verify backoff resets to minimum after a successful sync
- Verify concurrent multi-account failures all stay bounded
## Test plan
- [x] `task check-fast` passes locally
- [x] All 3 new tests pass with `fake_async` time control
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Summary
test/unit/reliability_runner_test.dartwith 3fake_asyncstress testsTest plan
task check-fastpasses locallyfake_asynctime control