Create order — API operation via OrderService

[rammrain]

Summary

Add a create method to OrderService that sends a POST /orders request with a CreateOrderRequest and returns a CreateOrderResponse.

Context

Issue #16 added OrderService.get(uuid) for retrieving orders. The CreateOrderRequest and CreateOrderResponse models already exist from #14. This issue covers wiring them through the service layer.

Requirements

• OrderService.create(CreateOrderRequest request) → CreateOrderResponse
• Validate request is not null before making the HTTP call
• Delegate to MontonioHttpClient.post("/orders", request, CreateOrderResponse.class)
• Unit tests with stubbed HTTP layer

Refs

• CreateOrderRequest — ee.bitweb.montonio.sdk.order.request
• CreateOrderResponse — ee.bitweb.montonio.sdk.order.response