fix: Resolve relative redirect locations

[DorianChn]

Summary

• Resolve relative redirect locations against the request URL that received the response, not the API base URL.
• Apply the behavior to 301 redirects and artifact, workflow-log, and archive download links.
• Preserve redirect validation and add regression coverage for rate-limited and non-rate-limited artifact downloads.

Why

Relative Location values are resolved against the responding request URL; using the API base URL can discard the endpoint path or return an unusable relative URL. This follows http.Response.Location.

Validation

• bash script/test.sh ./... — all 11 modules passed.
• go vet ./... — passed.
• Root custom golangci-lint — 0 issues; gofmt and git diff --check passed.
• script/lint.sh reached the generated-file check, which reports the local go.sum CRLF/LF mismatch; no unrelated line-ending change is included.

AI assistance

AI assistance helped draft the implementation and regression tests; the contributor reviewed the diff and ran the listed validations.

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.60%. Comparing base (4a73e54) to head (9210066).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
github/github.go	92.30%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4605   +/-   ##
=======================================
  Coverage   98.60%   98.60%           
=======================================
  Files         198      198           
  Lines       18552    18555    +3     
=======================================
+ Hits        18294    18297    +3     
  Misses        258      258           

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[gmlewis]

Is this PR fixing a hypothetical issue that has never been emperically found in the wild, or has this problem ever been actually seen when using this client library?

When would a relative URL ever be returned by GitHub?

How has this client library been working for decades without ever encountering a relative URL?