Async utilities count swallowed retryer failures as successes and call onSuccess(undefined)

[KevinVandy]

Describe the bug

AsyncRetryer.execute resolves undefined in three non-success cases: the retryer is disabled (enabled: false), all attempts failed with retryer-level throwOnError: false, or the execution was aborted.

Every async utility that wraps an internal retryer (AsyncDebouncer, AsyncThrottler, AsyncRateLimiter, AsyncQueuer, AsyncBatcher) treats that resolved undefined as a success: successCount increments, lastResult is overwritten with undefined, and onSuccess fires with undefined as the result — e.g. onSuccess: (result: string) => ... receives undefined at runtime.

Repro sketch:

const debouncer = new AsyncDebouncer(
  async () => { throw new Error('always fails') },
  {
    wait: 100,
    asyncRetryerOptions: { maxAttempts: 2, throwOnError: false },
    onSuccess: (result) => console.log('success?', result), // fires with undefined
  },
)

Design questions to settle

• Should a swallowed final failure settle as an error at the parent level (increment errorCount, fire onError) even though the retryer didn't throw?
• Should an aborted execution count as neither success nor error (probably — it already returns early in some paths)?
• AsyncRetryer.execute's undefined return is ambiguous by design; distinguishing outcomes may need an internal result envelope or reading the retryer's final state (lastError) after execute resolves.

Noted during review of #246 (see CodeRabbit feedback there) — split out because the fix changes observable semantics across all five utilities rather than being a bugfix in one.

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