add test for http proxy

[JacksonTian]

You need to complete

• unit tests and/or feature tests
• documentation is changed or added

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 26.31579% with 14 lines in your changes missing coverage. Please review.

Project coverage is 96.57%. Comparing base (b0bdc55) to head (84b8374).

Files	Patch %	Lines
credentials/internal/providers/oidc.go	0.00%	13 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #110      +/-   ##
==========================================
- Coverage   97.13%   96.57%   -0.56%     
==========================================
  Files          31       31              
  Lines        2092     2103      +11     
==========================================
- Hits         2032     2031       -1     
- Misses         42       53      +11     
- Partials       18       19       +1     

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