refactor: Extract inline coverage transform script (#11578)
## Summary
Extracts ~120 lines of inline Node.js from `turborepo-test.yml` into a
dedicated `scripts/transform-coverage.js` file.
**Before:** Coverage transformation logic was embedded directly in the
workflow YAML, making it untestable and hard to maintain.
**After:** Clean separation - the workflow calls the script with
input/output file paths. The script can now be tested independently.
