test(handler): expand handler test coverage 6.6% -> 44.9%
Added 40+ new handler tests covering:
- Auth: Register, Login (invalid JSON, missing company)
- Company: Get, Update, Delete, Verify, Rating, MyCompany
- Product: Get, Update, Delete (not found, invalid UUID)
- Inventory: List, Adjust (invalid params)
- Order: Create, Get, Update, Delete (validation)
- Payment: Preference, Webhook, Shipment
- Dashboard: Seller, Admin (authorization)
- User: CRUD operations (admin/seller scoping)
- Cart: Add, Get, Delete (context validation)
- Review: Create (validation)
Fixed MockRepository to return errors for not found entities
16a9ff7ffe