Reject invalid payment creation payloads

[bozicovichsantiago20-oss]

Summary

• Add a Zod payment creation schema requiring positive finite amounts.
• Normalize valid three-letter currency codes to lowercase and default missing currency to usd.
• Return a structured 400 response for invalid payment payloads before creating a mock intent.
• Update the API test script to target test files and add payment validation coverage.

Verification

• npm.cmd test -w apps/api
• node --test apps/api/src/tests/payment.test.js
• git diff --check

Closes #4181
Refs #743
/claim #4181