Fix utils.set preserving CSS background image URLs

[puneetdixit200]

Summary

• preserve url(...) tokens verbatim when decomposing complex CSS strings
• continue parsing numeric values outside URL tokens for interpolation
• add a regression test for utils.set() with backgroundImage URLs

Fixes #1154

Tests

• npm run build
• focused headless Chrome regression: red before fix, green after fix for utils.set(..., { backgroundImage })
• focused browser Mocha grep for Set CSS background image URL strings: 1 passing, 0 failing
• node --input-type=module source parser check for URL tokens and numeric values outside url(...)
• git diff --check

Local notes

• npm run test:node has exact V8 optimization-status assertion failures under local Node 24/22.
• Full headless browser Mocha completed with 594 passes and 5 unrelated engine/draggable/timing/leak failures in this environment.