[yuhuayang-google]

Summary

Fixes #194

In rest/nodejs, checkout creation and update previously accepted quantity: 0 and negative quantities because @ucp-js/sdk generated schemas in current published packages declare quantity as a bare z.number(). This allowed invalid quantities to flow through to totals math and create negative totals.

This change:

1. Enforces that line item quantity is an integer >= 1 in both createCheckout and updateCheckout.
2. Returns a standard UCP error envelope with code: "INVALID_REQUEST" and status 400 when an invalid quantity (negative, zero, or non-integer) is provided.
3. Adds integration test coverage in rest/nodejs/test/validation_flow.test.ts verifying that checkout create and update reject zero, negative, and fractional quantities.

Testing

• Ran npm test in rest/nodejs (all 165 tests pass).
• Ran npm run build (tsc) in rest/nodejs to ensure TypeScript compiles without error.

[damaz91]

Thank you so much for putting this together and adding thorough test coverage, @yuhuayang-google! 🙏

Issue #194 was recently resolved upstream at the Zod schema boundary via #199, which bumped @ucp-js/sdk to 0.4.5 (where LineItemCreateRequestSchema and LineItemUpdateRequestSchema enforce z.number().int().gte(1) and reject invalid quantities with 422 INVALID_REQUEST via prettyValidation, matching the Python reference implementation) and added regression coverage in rest/nodejs/test/quantity_bounds.test.ts.

Since the schema validation middleware now catches zero, negative, and non-integer quantities before reaching CheckoutService, I'll go ahead and close this PR as superseded by #199. Thanks again for contributing to UCP!