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.