test(request): cover request helpers

[pkurunner]

Summary

• add tests for request construction and header cloning
• cover JSON body encoding and error response parsing helpers

Testing

• gofmt -w internal/request/request_test.go
• GOOS=linux GOARCH=amd64 go test -c ./internal/request
• Not run: go test ./internal/request on Windows is blocked by Unix-only socket control APIs in
  awhttp.go.

Fixes #219