feat(async/unstable): add onRetry option to retry()

[tomas-zijdemans]

Adds an unstable onRetry callback to retry() so callers can log or measure retries without abusing isRetriable. Stable retry stays untouched.

Why

isRetriable(error) runs before the attempts-remaining check, so it also sees the terminal failure that becomes RetryError. It cannot tell whether another attempt will follow.

onRetry can. It fires after the retry decision is final, immediately before the backoff sleep.

API

onRetry?: (error: unknown, attempt: number, delay: number) => void;

• attempt is the 1-based number of the failed attempt.
• delay is the post-jitter delay passed to the sleep.
• The callback is synchronous. Its return value is ignored, and a throw stops retrying.
• It never fires for the terminal failure, an error rejected by isRetriable, or an already-aborted signal.

The callback observes a retry but cannot veto it or change its delay.

Review and testing

unstable_retry.ts follows the unstable-first pattern previously used for isRetriable and signal.

Ten tests cover arguments, timing, terminal failure, rejected errors, and abort behavior. deno task ok passes.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.99%. Comparing base (ca58f94) to head (d9d328c).

Files with missing lines	Patch %	Lines
async/unstable_retry.ts	60.00%	18 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7297      +/-   ##
==========================================
- Coverage   95.03%   94.99%   -0.05%     
==========================================
  Files         617      619       +2     
  Lines       51637    51957     +320     
  Branches     9359     9426      +67     
==========================================
+ Hits        49075    49355     +280     
- Misses       2021     2048      +27     
- Partials      541      554      +13     

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