📝 **Task:** Write `validate_order(payload)` returning `(True, '')` on success or `(False, msg)` on failure. Rules: `qty` must be present, an int, and > 0; `sku` must be present and a non-empty string. Return the FIRST failing rule's message in this exact priority: missing qty → 'qty required'; non-int qty → 'qty must be int'; qty <= 0 → 'qty must be > 0'; missing sku → 'sku required'; empty sku → 'sku must not be empty'. 📋 Implement the function above. Tests run automatically. 💡 **Hint:** Re-read the theory if stuck.