feat: validate restaurant_id against RestaurantRegistry in place_order

[AbdulmujibOladayo]

Summary

Validates restaurant_id against the RestaurantRegistry contract inside place_order() to prevent orders being placed against ghost or inactive restaurants.

Changes

• DataKey: Added RestaurantRegistry variant to store the registry contract address
• initialize(): Now accepts restaurant_registry_address: Address and stores it in instance storage
• place_order(): Before saving the order, makes a cross-contract call to RestaurantRegistry::get_restaurant(restaurant_id):
  • Panics with "restaurant not found" if the registry has no record (propagated from registry)
  • Panics with "restaurant is not active" if is_active is false
• Tests: All existing tests updated to pass registry_id to initialize(). Added MockRestaurantRegistry for unit tests with:
  • ID 1: active restaurant (happy path)
  • ID 2: inactive restaurant
  • Any other ID: panics with "restaurant not found"
  • New tests: valid active restaurant passes, inactive restaurant panics, nonexistent restaurant panics

Closes #282