fix(quota): correct breaker halfOpen comment and add concurrent test

[vishalmore90]

Ⅰ. Describe what this PR does

This PR corrects a factually wrong comment in BreakerBackend.beforeCall() that misrepresents the state machine, and introduces a missing concurrent unit test to cover this specific race condition.

Context:
In pkg/sandbox-manager/quota/breaker.go line 118, the comment described the halfOpen == true state as: // 3. the probe acquire failed, breaker stays open.

This is incorrect. When beforeCall() executes the halfOpen == true branch, it means a probe acquire is currently in-flight in another goroutine, not that it has failed. The actual failure path is handled asynchronously inside afterCall(), which re-opens the breaker.

Changes:

1. Comment Fix: Updated the comment in breaker.go to accurately describe the fail-fast thundering herd protection.
2. Missing Test Added: Added a new test concurrent caller during in-flight probe gets ErrBackendUnavailable without touching inner to breaker_test.go. This test uses a new blockingBackend to intentionally pause the probe mid-flight and ensure that a concurrent caller correctly fails-fast with ErrBackendUnavailable without incrementing the inner backend call count.

Ⅱ. Does this pull request fix one issue?

fixes #897

Ⅲ. Describe how to verify it

1. Review the logic trace in breaker.go to confirm the comment change aligns with the afterCall() state transitions.
2. Run the newly added test suite to verify the race condition is properly protected against:

   go test -v -run TestBreaker ./pkg/sandbox-manager/quota

[kruise-bot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.40%. Comparing base (825de1d) to head (970839c).
⚠️ Report is 21 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #898      +/-   ##
==========================================
+ Coverage   82.31%   82.40%   +0.08%     
==========================================
  Files         244      254      +10     
  Lines       19782    21457    +1675     
==========================================
+ Hits        16284    17681    +1397     
- Misses       2888     3079     +191     
- Partials      610      697      +87     

Flag	Coverage Δ
unittests	82.40% <ø> (+0.08%)	⬆️

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