[aws2] Fix flaky SqsIOWriteBatchesTest timeout tests (#38946)

[tkaymak]

What

Fixes the flaky SqsIOWriteBatchesTest (#38946).

Why

The four timeout-related tests:

• testWriteBatchesWithTimeout
• testWriteBatchesWithStrictTimeout
• testWriteBatchesToDynamicWithTimeout
• testWriteBatchesToDynamicWithStrictTimeout

asserted the exact grouping of messages into SendMessageBatch calls.
Those groupings depend on wall-clock timing, the per-message Thread.sleep delay racing the configured withBatchTimeout, so on loaded CI runners the batches form differently and the strict verify(sqs).sendMessageBatch(request(exact entries)) checks fail with Mockito ArgumentsAreDifferent.

How

Replace the exact-grouping assertions with timing-independent invariants:

• every expected message body is sent exactly once (captured via ArgumentCaptor, grouped by queue),
• no batch exceeds the size implied by the timeout cadence,
• at least the minimum number of batches is produced (verify(sqs, atLeast(n))).

This still exercises timeout-driven flushing (both the synchronous on-append path and the strict separate-thread variant) without depending on exact wall-clock behavior. The non-timeout tests are unchanged.

Testing

Ran the class 6 times under deliberate 4-core CPU saturation (load avg ~12) with no failures; previously the timeout tests would flake under load.

Fixes #38946

[gemini-code-assist]

Summary of Changes

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This pull request addresses flaky tests in SqsIOWriteBatchesTest caused by reliance on wall-clock timing for batch grouping assertions. By shifting from strict order-based verification to invariant-based verification, the tests are now resilient to varying execution speeds on loaded CI runners while still effectively validating the timeout-driven flushing logic.

Highlights

• Test Stability: Replaced brittle, timing-dependent Mockito assertions in SqsIOWriteBatchesTest with robust, timing-independent invariants to prevent flakiness in CI environments.
• Assertion Logic: Introduced helper methods captureBatchRequests and assertMessageBodies to verify that all expected messages are sent exactly once and that batch sizes remain within expected limits, regardless of exact execution timing.

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[gemini-code-assist]

Code Review

This pull request refactors several tests in SqsIOWriteBatchesTest.java to use timing-independent invariants instead of strict timing assertions, which helps prevent test flakiness on loaded machines. The feedback focuses on resolving potential Checker Framework nullness warnings by properly wrapping nullable objects (such as the results of Map.get and the @Nullable parameter in assertMessageBodies) with checkNotNull before dereferencing or passing them to assertions.

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[github-actions]

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