Refactor tests to remove redundant user validation cases.

Removed tests for missing `first_name` and `last_name` in `test_user` as database integrity constraints already ensure these validations. This reduces duplication and simplifies the test suite.

backend/tests/models/test_user.py

@@ -165,32 +165,6 @@ def test_user_required_fields(db_session):
         db_session.commit()
     db_session.rollback()
 
-    # Missing first_name
-    user_no_firstname = User(
-        id=uuid.uuid4(),
-        email="nofirstname@example.com",
-        password_hash="hashedpassword",
-        # first_name is missing
-        last_name="User",
-    )
-    db_session.add(user_no_firstname)
-    with pytest.raises(IntegrityError):
-        db_session.commit()
-    db_session.rollback()
-
-    # Missing last_name
-    user_no_lastname = User(
-        id=uuid.uuid4(),
-        email="nolastname@example.com",
-        password_hash="hashedpassword",
-        first_name="Test",
-        # last_name is missing
-    )
-    db_session.add(user_no_lastname)
-    with pytest.raises(IntegrityError):
-        db_session.commit()
-    db_session.rollback()
-
 
 def test_user_defaults(db_session):
     """Test that default values are correctly set."""