bug: Mentor registration unsubmittable when UI-optional technical/focus areas are empty (backend @NotEmpty)

[yingliu-data]

Bug Report

Mentor registration cannot be submitted when the user leaves "technical areas" or "mentorship focus areas" empty — even though the UI presents these as optional — because the backend requires them.

• Frontend src/schemas/mentorSchema.ts:107-134 declares technicalAreas and mentorshipFocusAreas as .optional().default([]), and the step components (Step3DomainSkills.tsx, Step4ProgrammingSkills.tsx) tell the user none are mandatory.
• Backend domain/platform/mentorship/Skills.java declares both areas and mentorshipFocus as @NotEmpty, and MentorDto.skills is @NotNull @Valid.

Reproduction

A mentor completes the 5-step form selecting no technical areas (explicitly allowed by the UI) and submits → POST /api/platform/v1/mentors returns 400 with a validation error → the submission fails after the user has filled in the entire form.

The failure is made worse by the error-handling bug where the registration page reads errorData.error while the backend returns { message }, so the user sees only a generic "Failed to submit application" with no hint about the empty required fields.

Severity

High

Potential Risk

• Mentors who follow the UI's guidance cannot register at all; drop-off after completing a long multi-step form.

Suggested Fix

Decide the intended contract and align both ends:

• If these fields are truly required: make them required in the Zod schema and mark the steps as mandatory with inline validation before the final submit.
• If they should be optional: relax the backend @NotEmpty constraints on Skills.areas/mentorshipFocus (and adjust any dependent logic).
• Separately, fix the registration error handler to read body.message so the real validation error is surfaced.
• Add an end-to-end test for a minimal mentor registration matching whichever contract is chosen.