test(pubsub): add unit tests for publish hedging queue integration

[feywind]

Overview

This PR is Layer 6 of 7 in Stack #9467, which implements Publish Hedging for @google-cloud/pubsub to mitigate high-percentile (p99/p99.9) publish tail latencies by sending backup (hedged) publish RPCs when an in-flight batch takes longer than a configurable hedgeDelay threshold.

Stack Structure

1. feat(gax): support cancelling API calls via CallOptions.signal #9462 - feat(gax): support AbortSignal cancellation in CallOptions and createApiCall
2. feat(pubsub): add HedgingOptions validation and token bucket #9463 - feat(pubsub): add HedgingOptions validation and HedgingTokenBucket
3. feat(pubsub): add CancellationSharer and HedgingScheduler #9464 - feat(pubsub): add CancellationSharer and HedgingScheduler
4. feat(pubsub): add OpenTelemetry events and logging for hedged publish #9465 - feat(pubsub): add OpenTelemetry events and logging for hedged publish
5. feat(pubsub): integrate publish hedging into MessageQueue #9466 - feat(pubsub): integrate publish hedging into MessageQueue
6. test(pubsub): add unit tests for publish hedging queue integration #9635 - test(pubsub): add unit tests for publish hedging queue integration (this PR)
7. test(pubsub): add publish hedging system tests #9636 - test(pubsub): add publish hedging system tests

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Summary of Changes in This PR

This layer adds unit test coverage for the publish hedging queue integration and AbortSignal handling in PubSub.prototype.request:

• handwritten/pubsub/test/publisher/message-queues.ts:
  • Default hedging behaviour when no hedging occurs.
  • Original attempt (attempt 0) winning after a hedged attempt is dispatched, aborting the hedged attempt via AbortSignal and recording publish end.
  • Hedged attempt winning first, aborting attempt 0 and recording publish end (hedged).
  • Hedged attempt errors being ignored while attempt 0 succeeds.
  • Terminal failure of attempt 0 rejecting the batch and aborting any in-flight hedged attempts.
  • Multiple sequential hedged attempts (attempt 1, attempt 2, ...) at hedgeDelay intervals.
  • Token bucket exhaustion ('Hedging rate limited due to lack of tokens.') and token replenishment over subsequent successful publishes.
  • Ensuring OrderedQueue never hedges.
• handwritten/pubsub/test/pubsub.ts:
  • Pre-aborted AbortSignal before getClient_ completes and AbortSignal aborted while getClient_ is in flight.

[gemini-code-assist]

Code Review

This pull request introduces comprehensive unit tests for publish hedging in message queues, refactors existing tests to use async/await, and adds tests to verify that pubsub.request correctly handles aborted signals. The review feedback highlights critical runtime ReferenceErrors in the new tests due to missing namespace qualifiers on Queue and OrderedQueue, and recommends replacing the generic Function type with a more specific callback signature in the pubsub tests.