Add Zod input validation to Product API POST endpoint

[Copilot]

POST /api/products accepted any payload without validation, allowing products to be created with missing or invalid required fields.

Changes

api/src/routes/product.ts

• Added a productSchema using Zod that enforces:
  • name — non-empty string
  • price — positive number
  • supplierId — positive integer
• POST / now returns 400 with structured error details on validation failure; 201 on success
• Exported resetProducts() for test isolation (mirrors branch.ts pattern)

api/src/routes/product.test.ts (new)

• Tests covering valid creation and each missing/invalid required field case

api/package.json

• Added zod v4 dependency

Example

// Missing supplierId → 400
POST /api/products  { name: "Widget", price: 9.99 }
// → 400 { error: "Validation failed", details: [...] }

// All required fields present → 201
POST /api/products  { name: "Widget", price: 9.99, supplierId: 3, ... }
// → 201 { ...product }