fix(async/unstable): circuit breaker stale openedAt and duplicate callbacks

[tomas-zijdemans]

• Fix cooldown timer bypassed when wrapped function is slow
• Fix duplicate callbacks on concurrent half-open failures
• Extract tryCall helper to simplify predicate error handling
• Improve JSDoc and add test for synchronous throws

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.42%. Comparing base (a3a6ef6) to head (226dff4).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7097      +/-   ##
==========================================
- Coverage   94.43%   94.42%   -0.01%     
==========================================
  Files         630      630              
  Lines       50566    50566              
  Branches     8969     8973       +4     
==========================================
- Hits        47750    47749       -1     
  Misses       2247     2247              
- Partials      569      570       +1     

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[bartlomieju]

LGTM — both bug fixes are real and correctly addressed.

Stale openedAt: currentTime was captured at the top of execute() and passed into #handleFailure as openedAt. If fn() is slow, the cooldown starts from the past. Using Date.now() at failure time is the right fix.

Duplicate callbacks: When halfOpenMaxConcurrent > 1 and multiple requests fail concurrently, each #handleFailure call saw previousState === "half_open" and fired onStateChange/onOpen. Checking the current state to gate callbacks and preserving the existing openedAt is correct.

The tryCall extraction is clean. One subtlety: it only wraps the predicate, not #handleFailure (which the old try/catch did), but that's fine since #handleFailure can't throw — all callbacks go through safeCallback.

Nit: the @returns void tags on forceOpen, forceClose, reset are unnecessary — void return is self-evident from the signature.