fix(drizzle-kit): handle commas in SQLite expression index columns

[mixelburg]

squashIdx and squashUnique used comma-joined column strings which broke when an expression column contained its own comma (e.g., json_extract(payload, '$.ref')). This caused to emit invalid SQL with backtick-quoted expression fragments.\n\nUses JSON encoding for the columns array with a split(',') fallback for backward compatibility with existing snapshot files.\n\nFixes #6062

[skdas20]

Nice approach — fixing it at the serialization boundary rather than trying to re-join fragments downstream is the right call, since the isExpression guard in the convertor can never match once the string has already been split.

One edge case in the backward-compat path, though. The fallback only fires on a thrown parse error:

try {
    columns = JSON.parse(columnsString);
} catch {
    columns = columnsString.split(',');
}

For an old-format snapshot, columnsString is the raw comma-joined names. Most of the time that isn't valid JSON so it throws and the fallback is correct. But if the index has a single column whose name happens to be valid JSON, JSON.parse succeeds and returns a non-array:

"123"   -> 123      (number)
"true"  -> true     (boolean)
"null"  -> null
"1e5"   -> 100000

So an old snapshot with an index on a column named 123 (legal as a quoted identifier) silently yields columns: 123 instead of ["123"], skipping the fallback entirely. It'll then blow up in index.parse as a zod error rather than falling back gracefully.

Guarding on the shape rather than on throwing handles it:

let columns: string[];
try {
    const parsed = JSON.parse(columnsString);
    columns = Array.isArray(parsed) ? parsed : columnsString.split(',');
} catch {
    columns = columnsString.split(',');
}

Same applies to the unsquashUnique / unsquashPK copies of this pattern if they take the same shape.

Might also be worth a test with an old-format squashed string as input, since the current tests presumably all round-trip through the new squashIdx and so never exercise the fallback branch at all.