Adds a new education entry to the candidate's profile and returns the updated Candidate object. The new entry is appended to the candidate's existing education history rather than replacing it. school_name is required; field_of_study, start_year, and end_year are optional (the request body's flat start_year/end_year are stored as the nested start_date: { year } / end_date: { year } shape seen on education entries in the Candidate response).