fix(async/unstable): make circuit breaker state getter pure

[tomas-zijdemans]

Fixed an issue where accessing .state can mutate internal state and fire onStateChange callback.

Also added a note documenting a known race condition

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.18%. Comparing base (45e2b13) to head (b705046).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6926      +/-   ##
==========================================
- Coverage   94.18%   94.18%   -0.01%     
==========================================
  Files         584      584              
  Lines       43124    43124              
  Branches     6896     6896              
==========================================
- Hits        40618    40617       -1     
- Misses       2455     2456       +1     
  Partials       51       51              

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[kt3k]

LGTM