Checkout Service: Implement discount validation and price adjustment

[rishikeshradhakrishnan]

Parent Issue

Part of #96 — Add promotional discount code feature to checkout flow

Service

src/checkout (Go)

Scope

Discount Code Store

• Create an in-memory discount code store with the following codes:

  Code	Type	Rule
  SAVE10	Percentage	10% off entire order subtotal
  FREESHIP	Shipping	Set shipping cost to $0
  ASTRO20	Conditional	20% off when subtotal > $100

• Only one discount code allowed per order

Validation Logic

• Validate the discount code from PlaceOrderRequest
• For ASTRO20, check that the subtotal (after currency conversion) exceeds $100 equivalent
• Return appropriate error if code is invalid or conditions are not met

Price Adjustment in PlaceOrder()

• Apply discount after currency conversion but before calling the Payment service
• For percentage discounts: calculate discount on item subtotal (excluding shipping)
• For FREESHIP: zero out the shipping cost from the Shipping service quote
• Pass the adjusted total to the Payment service Charge() call
• Include discount details in the OrderResult response

OpenTelemetry Instrumentation

• Add a span for discount code validation (discount.validate)
• Record span attributes: discount.code, discount.type, discount.amount, discount.valid
• Add a metric counter for discount code usage (by code and success/failure)

Acceptance Criteria

• In-memory store holds all three discount codes
• SAVE10 correctly applies 10% off the item subtotal
• FREESHIP sets shipping to $0
• ASTRO20 applies 20% off only when subtotal > $100; returns error otherwise
• Invalid codes return a descriptive gRPC error
• Payment service receives the correctly adjusted total
• OrderResult includes discount information
• OpenTelemetry spans and metrics are recorded for discount operations