fix(async/unstable): ignore stale half-open requests in circuit breaker

[tomas-zijdemans]

A request admitted during one half-open period could finish after the circuit had re-opened and gone half-open again. Its success then counted toward the new period, and its cleanup freed a slot the new period never handed out, so a probe got through above halfOpenMaxConcurrent.

Each OPEN → HALF_OPEN transition now bumps a generation counter. A request records the generation at admission, and both its success handling and its in-flight decrement are skipped when the generation has moved on.

Two tests cover the stale close and the leaked slot. Both fail on main.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.03%. Comparing base (ca58f94) to head (1aa0e99).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7306      +/-   ##
==========================================
- Coverage   95.03%   95.03%   -0.01%     
==========================================
  Files         617      618       +1     
  Lines       51637    51899     +262     
  Branches     9359     9409      +50     
==========================================
+ Hits        49075    49323     +248     
- Misses       2021     2030       +9     
- Partials      541      546       +5     

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