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Summary

Fixes the flaky ThreadPendingSendDrainerTest.timeoutExceptionIsClassifiedAndBackedOff (task #2 in #wg-mobile). It failed once in #3373's first CI run (expected [random-1, random-1]), and a rerun passed.

Cause (test only): these tests drive drain passes by hand with drainOnceForTests. When a test didn't pass a wakeups capture list, the helper left the real backoff scheduler armed. After a retryable failure (timeout, 5xx), that real wakeup launched a second drain pass on Dispatchers.Default, and it raced the backoff pass the test runs itself. Both passes could send the row, so send.randomIds became [random-1, random-1, random-1].

Production is not affected. In production every pass, including backoff wakeups, goes through the serial consumer loop (trigger → consumeLoop), so two passes never run at once.

Change:

• The test helper now always captures wakeups (wakeups defaults to a fresh list), so no unit test arms a real timer.
• The timeout test asserts that it armed exactly one 0ms wakeup, like the 429 retry test.

Verification

• Reproduced before the fix: running the timeout scenario in a 2000× loop locally hit the triple send.
• After the fix: 2000/2000 iterations passed, and the whole ThreadPendingSendDrainerTest class passed (14 tests, :shared:testDebugUnitTest). The loop was temporary and is not committed.

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