[release/6.0] Fixed duplicate metadata in ModelDataSource (#36340)

* Fixed duplicate metadata in ModelDataSource - Today when we make endpoints for the ModelDataSource, we apply the callbacks to the EndpointBuilder before creating the endpoint. This happens every time anyone accesses the endpoints property. Instead of re-applying the conventions, we do it once on Build. We also throw if more conventions are added after building. - Added tests

* Apply suggestions from code review

Co-authored-by: Pranav K <prkrishn@hotmail.com>

Co-authored-by: David Fowler <davidfowl@gmail.com>
Co-authored-by: Pranav K <prkrishn@hotmail.com>