Prevent nil request panic during resolve

[Copilot]

Resolve requests with missing req caused a nil-pointer panic in extension URL matching, preventing downloads in Docker deployments.

• Request validation
  • Return a descriptive error when Downloader.Resolve receives a nil request.
• Nil-safe extension matching
  • Make Script.match reject nil requests before accessing URL or labels.
• Regression coverage
  • Add tests for nil resolve requests and nil script-match inputs.

if req == nil {
    return nil, errors.New("request cannot be nil")
}

• Fixes 1.9.3 docker版下载不了东西，提示：runtime error: invalid memory address or nil pointer dereference #1422