fix(gax): panic in LimitedAttemptCount

[coryan]

Fixes #6677

[gemini-code-assist]

Code Review

This pull request updates the on_throttle method in LimitedAttemptCount to handle cases where the inner policy continues but the maximum attempt count has been reached, returning ThrottleResult::Exhausted instead of asserting. It also adds a corresponding unit test. The review feedback suggests simplifying the nested match and conditional logic using a match guard to improve readability and adhere to the style guide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.67%. Comparing base (86c2a65) to head (c9d1217).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6725   +/-   ##
=======================================
  Coverage   96.66%   96.67%           
=======================================
  Files         308      308           
  Lines       97778    97801   +23     
=======================================
+ Hits        94519    94546   +27     
+ Misses       3259     3255    -4     

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