[lazerg]

Summary:

Two bugs in the backgroundImage string parser. getPositionFromCSSValue only accepted px and %, so a unitless 0 stop like linear-gradient(red 0, blue 100%) was treated as an invalid position and the whole gradient was dropped. CSS allows a bare 0 as a length, so it now parses to 0.

Also, when one layer of a multi-layer string failed to parse, parseBackgroundImageCSSString skipped it and kept the others. In CSS one invalid layer invalidates the whole value, and the array form here already returns [] in that case, so the string form now does the same.

Fixes #59603

Changelog:

[GENERAL] [FIXED] - Accept unitless 0 color stop positions and drop the whole value when one layer is invalid in backgroundImage strings

Test Plan:

Added two cases to processBackgroundImage-itest.js. Both fail on main and pass with this change, and the rest of the file still passes. I ran the file under plain Jest (without the Fantom environment import) since I don't have the Fantom runner built locally. Prettier, ESLint and Flow are clean.