fix(async/unstable): evaluate circuit breaker failure rate on success

[tomas-zijdemans]

A success in closed state returned before the failure rate was checked. When that success was the request that lifted the window to minimumThroughput, the circuit stayed closed even though the rate already qualified. With minimumThroughput: 3 and failureRateThreshold: 0.5, failure, failure, success left the circuit closed at a 67% failure rate. The docs say it opens.

Closed-state successes now run the same rate check as failures, and open the circuit with the usual onStateChange and onOpen callbacks when it qualifies. The check is factored into one helper so both paths agree.

One test covers the trip, one covers a success that keeps the rate below the threshold.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.03%. Comparing base (ca58f94) to head (0a4f638).

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #7308    +/-   ##
========================================
  Coverage   95.03%   95.03%            
========================================
  Files         617      618     +1     
  Lines       51637    51909   +272     
  Branches     9359     9412    +53     
========================================
+ Hits        49075    49334   +259     
- Misses       2021     2030     +9     
- Partials      541      545     +4     

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