Resolve local and quoted CSS asset URLs safely

[OskarEichler]

Summary

• preserve the credited Fix asset url processor regex for where subdirectory starts with 'data' #543 correction so local names beginning with data or http are fingerprinted while actual URI schemes remain unchanged
• parse quoted CSS url() values without truncating spaces or closing parentheses
• leave empty, fragment, protocol-relative, and arbitrary scheme references unchanged

Verification

• cumulative unmodified suite: 151 tests / 796 assertions on Ruby 4.0.6 + Rails 8.1/Sprockets 4, Ruby 3.2.11 with the same stack, and Ruby 3.2.11 + Rails 7.2/Sprockets 3
• cumulative external models: 28 checks per configuration, including quoted spaces, parentheses, schemes, fragments and local paths
• syntax, package parity, bundle and backend integration checks pass

The earlier credited #543-derived commit and its existing regression tests remain on this branch. This follow-up changes runtime source only; no test file was changed in this audit pass.