Refactor pricing function + fix quantity=0 edge case + add tests

[Slackmom]

Context:
The current pricing logic is hard to read and risky to change. There is a known bug: quantity=0 should return a total of 0, but the function currently charges for at least one item.

Goal:
Refactor the pricing logic into small, pure functions and fix the quantity=0 edge case.

Non-goals:

• No new features
• No API/signature changes for calcPrice(input)
• No changes outside /src/pricing/** and /tests/**

Acceptance Criteria:

1. calcPrice keeps the same input/output shape
2. Logic is split into small functions (readable + testable)
3. Tests cover: basic case, quantity=0 edge case, and one regression scenario
4. npm test passes
5. PR description includes: Summary, Risk/Notes, How to test

Constraints:

• Touch only: /src/pricing/** and /tests/**
• Keep the change reviewable (small steps)

How to test:

• Run npm test
• Example inputs:
  • quantity=1, DE, no coupon
  • quantity=0 (edge case)