refactor(async/unstable): apply style guide to circuit breaker

[tomas-zijdemans]

• Added @experimental tags to all public exports
• Fixed error message style (double quotes around parameter name,, colon separator, no trailing period +++)
• Avoid @param unless parameter intent is non-obvious

Also removed overlapping tests. Test coverage is still 100%.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.16%. Comparing base (4936f23) to head (897fb9c).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7008   +/-   ##
=======================================
  Coverage   94.16%   94.16%           
=======================================
  Files         618      618           
  Lines       48802    48802           
  Branches     8601     8601           
=======================================
+ Hits        45954    45956    +2     
+ Misses       2779     2777    -2     
  Partials       69       69           

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[kt3k]

Nice! LGTM