Derive the union of index token names from a values-first config input.
When the provided config literal carries an indexes object with preserved
literal keys (prefer as const at call sites), this helper captures the
index token union. Falls back to string if absent.
Derive the union of index token names from a values-first config input.
When the provided config literal carries an
indexesobject with preserved literal keys (preferas constat call sites), this helper captures the index token union. Falls back tostringif absent.