Upsert Schema Fields

Creates or updates user-defined fields on a schema, matched by field_id. Fields are matched by `field_id`. Reusing an existing `field_id` updates that field; using a new `field_id` creates a new user-defined field. This makes the endpoint safe to retry when you are intentionally upserting the same field definitions. It is not a patch-by-position operation. If some fields succeed and others fail, the endpoint can return a partial success response. Validate the response status and message rather than assuming the whole batch was applied uniformly.