feat(auth): add retry loop for generate_access_token

[alvarowolfx]

Towards #4459

[gemini-code-assist]

Code Review

This pull request refactors the generate_access_token function in src/auth/src/credentials/impersonated.rs to use a new GenerateAccessTokenClient struct, integrating a retry loop with exponential backoff and strict AIP-194 retry policies from google-cloud-gax. It also adds corresponding unit tests to verify successful retries and exhausted retry attempts. Feedback on the changes highlights an issue where formatting err_payload (of type bytes::Bytes) with {:?} results in a non-human-readable debug representation of byte integers. The reviewer suggests passing the raw payload directly to GaxError::http to preserve the response body and avoid redundant error message wrapping.

[gemini-code-assist]

Formatting err_payload (which is of type bytes::Bytes) using {:?} inside format! will output a debug representation of the bytes (e.g., a list of integers like [123, 34, ...]), which is not human-readable and makes debugging HTTP errors extremely difficult.

Additionally, prepending {MSG} here is redundant because the caller (fetch) already wraps the returned error using errors::from_gax_error(e, MSG).

We should pass the raw err_payload directly to GaxError::http to preserve the raw HTTP response body (e.g., JSON error details) so that it can be properly formatted or parsed later.

Suggested change

	return Err(GaxError::http(
	status.as_u16(),
	err_headers,
	format!("{MSG}: {err_payload:?}").into(),
	));
	return Err(GaxError::http(
	status.as_u16(),
	err_headers,
	err_payload,
	));

[codecov]

Codecov Report

❌ Patch coverage is 99.29577% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.04%. Comparing base (2f59820) to head (4aec5eb).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/auth/src/credentials/impersonated.rs	99.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6092      +/-   ##
==========================================
- Coverage   97.04%   97.04%   -0.01%     
==========================================
  Files         253      253              
  Lines       63682    63793     +111     
==========================================
+ Hits        61798    61905     +107     
- Misses       1884     1888       +4     

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