client: surface last-response details after retries are exhausted

[alliasgher]

Summary

When retryablehttp exhausts retries, its default failure message is just:

<METHOD> <URL> giving up after N attempt(s)

If the last failure was a transport error the detail survives via %w wrapping, but if the server returned a non-retryable error response (e.g. 5xx with a body explaining the problem), retryablehttp drains and discards the body — users see no root cause. That's the exact scenario reported in #2640.

This PR installs a custom retryablehttp.ErrorHandler on the Rekor client so the final error includes:

• The wrapped transport error when one is present, or
• The final HTTP status code and a bounded (512-byte) body snippet from the response.

Fixes #2640

Test plan

• go test ./pkg/client/
• gofmt clean
• New TestRetryErrorHandlerSurfacesServerResponse: runs the client against a test server returning 500 + body "database unavailable"; asserts the final error contains both status 500 and the body text.

Signed-off-by: Ali alliasgher123@gmail.com

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 25.22%. Comparing base (488eb97) to head (ea377d9).
⚠️ Report is 670 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/client/rekor_client.go	62.50%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2796       +/-   ##
===========================================
- Coverage   66.46%   25.22%   -41.24%     
===========================================
  Files          92      191       +99     
  Lines        9258    25382    +16124     
===========================================
+ Hits         6153     6402      +249     
- Misses       2359    18151    +15792     
- Partials      746      829       +83     

Flag	Coverage Δ
e2etests	46.22% <6.25%> (-1.34%)	⬇️
unittests	16.30% <62.50%> (-31.38%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.