Add unit tests for custom error types

[GautamBytes]

This pull request introduces unit tests for the custom error types RetryError and BadRequestError defined in pkg/util/errors/types.go.

Purpose:

The tests are added to validate the behavior of our custom error handling, ensuring that:

• The Error() methods produce the correct string output.
• The Is... and As... helper functions correctly identify and unwrap custom errors, even when they are wrapped by other errors using fmt.Errorf with %w.

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[GautamBytes]

@hantmac please review it whenever feasible!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.65%. Comparing base (00aae12) to head (1d9cbc8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #289      +/-   ##
==========================================
+ Coverage   47.52%   47.65%   +0.12%     
==========================================
  Files          63       64       +1     
  Lines        9799     9823      +24     
==========================================
+ Hits         4657     4681      +24     
  Misses       4585     4585              
  Partials      557      557              

Flag	Coverage Δ
unittests	47.65% <ø> (+0.12%)	⬆️

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[GautamBytes]

#288 (comment)
same here !