The withSchema method is documented to set the default schema for all table references that don't explicitly specify a schema (docs). However, it currently fails to apply the schema to join tables in mergeInto statements.

When using withSchema with mergeInto, the schema is not applied to the table specified in the .using() clause:

await db
 .withSchema(tenant)
 .mergeInto('person as target')
 .using('pet as source', 'source.owner_id', 'target.id')
 .whenMatchedAnd('target.has_pets', '!=', 'Y')
 .thenUpdateSet({ has_pets: 'Y' })
 .whenNotMatchedBySourceAnd('target.has_pets', '=', 'Y')
 .thenUpdateSet({ has_pets: 'N' })
 .executeTakeFirstOrThrow()

In this example, the person table correctly receives the schema prefix, but the pet table in the .using() clause does not.

This PR updates the WithSchemaTransformer class to properly handle join tables in mergeInto statements.