fix(http): keep request URL normalization reusable

[ryanfowler]

Summary

• validate missing request URLs instead of panicking during request construction
• normalize schemes and query parameters on a per-request URL copy so repeated requests do not accumulate state
• retain the effective URL privately for post-failure diagnostics and recognize Go 1.27 HTTP/HTTPS scheme mismatches
• add unit and integration coverage for request reuse and plaintext-service hints

Testing

• go test -v -parallel 8 ./...
• go test -race ./internal/client ./internal/fetch
• go build .
• CGO_ENABLED=0 go build .
• go mod tidy && go mod verify
• staticcheck ./...