chore: update basic auth error status

[zhaohuabing]

This PR makes the “basic auth secret not found” error clearer to avoid confusion (e.g., #8588).

[netlify]

✅ Deploy Preview for cerulean-figolla-1f9435 canceled.

Name	Link
🔨 Latest commit	6d85e22
🔍 Latest deploy log	https://app.netlify.com/projects/cerulean-figolla-1f9435/deploys/69c35ccaf8d15800086da34e

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.34%. Comparing base (a5535c4) to head (6d85e22).

Files with missing lines	Patch %	Lines
internal/gatewayapi/securitypolicy.go	0.00%	2 Missing ⚠️

❌ Your patch check has failed because the patch coverage (0.00%) is below the target coverage (60.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8589      +/-   ##
==========================================
- Coverage   74.40%   74.34%   -0.07%     
==========================================
  Files         243      243              
  Lines       38143    38143              
==========================================
- Hits        28381    28357      -24     
- Misses       7776     7799      +23     
- Partials     1986     1987       +1     

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