findIndex in src/client/adapter-utils.ts never matches composite indexes

[nicorne]

Bug: Underscore prefix on range/sort fields breaks composite index lookup

Lines 190 and 195 prepend _ to the range and sort field names when building the lookup array:

// L190 (range)
`${indexEqFields.length ? "_" : ""}${boundField}`

// L195 (sort)
`${indexEqFields.length || boundField ? "_" : ""}${sortField}`

This produces ["organizationId", "_expiresAt"], which is then compared field-by-field against the index definition ["organizationId", "expiresAt"] at L211:

indexFields.every((field, idx) => field === fields[idx])

"_expiresAt" !== "expiresAt" — the index is never found, falling back to a full table scan.

Why it wasn't caught: Auto-generated Better Auth indexes are single-field (indexEqFields.length === 0), so no underscore is added. The bug only surfaces with custom composite indexes (2+ fields).

Fix — remove the underscore prefixing (L190, L195):

- .concat(boundField && boundField !== "createdAt"
-   ? `${indexEqFields.length ? "_" : ""}${boundField}`
+ .concat(boundField && boundField !== "createdAt"
+   ? boundField
    : "")
- .concat(sortField && sortField !== "createdAt" && boundField !== sortField
-   ? `${indexEqFields.length || boundField ? "_" : ""}${sortField}`
+ .concat(sortField && sortField !== "createdAt" && boundField !== sortField
+   ? sortField
    : "")