* Add extraBody field to model configuration forms
This adds a new field allowing users to specify additional JSON fields
to inject into the request body when configuring models.
* Handle ExtraBody clearing when frontend sends empty object
The backend now interprets an empty object sent from the frontend as a
signal to clear the ExtraBody field, while nil/undefined preserves the
existing value. Frontend changed to send {} instead of undefined when
the field is empty.
3b3062abe8