Improve the error message when a required field from the API is not provided

Use pydantic `is_required()` and ask users to fill in before sending requests or building it. Auto-generate appropriate messages
Combine with #57634 if possible

Start by locating the API validation path that handles required fields and inspect how Pydantic's is_required() is currently used. Compare the intended scope with issue #57634; done means missing required fields produce automatically generated messages telling users to fill them before sending requests or building the object.

Written by the indexing model from the issue text.