What?

This PR fixes invalid gap CSS generation when styles.spacing.blockGap in theme.json uses nested var() fallbacks, and the user updates row/column gap controls in the editor.
Closes #76896

Why?

The previous fallback logic split processed gap strings by whitespace. That approach breaks values like:

"blockGap": "var(--a, var(--b, 1.2rem))"

because whitespace inside nested var() expressions is not a safe delimiter.

How?

Use existing gap helpers to read row/column values directly instead of string splitting:

• getGapBoxControlValueFromStyle(...)
• getSpacingPresetCssVar(...)

This keeps the fix minimal and scoped to fallback resolution.

Testing Instructions

1. Use a block theme with:

   {
     "styles": {
       "spacing": {
         "blockGap": "var(--a, var(--b, 1.2rem))"
       }
     }
   }

2. Insert a Columns block.
3. Confirm initial gap is valid.
4. Update only column-gap (and then row-gap) from block settings.
5. Verify generated CSS remains valid and parentheses stay balanced.