fix(rest/nodejs): block completion when fulfillment methods array is empty

[XiaolongZhang-TT]

Description

The Node.js completion gate used checkout.fulfillment?.methods?.every(...), which is vacuously true when methods is an empty array. A checkout created with fulfillment: { methods: [] } therefore passed the "fulfillment selected" check and could be completed (order placed) with no fulfillment at all — while a checkout with no fulfillment field is correctly rejected.

Repro: POST /checkout-sessions with fulfillment: { methods: [] } returns 201, then POST /checkout-sessions/{id}/complete currently returns 200 (order placed, no fulfillment).

Root cause — rest/nodejs/src/api/checkout.ts, completeCheckout:

const hasFulfillment = checkout.fulfillment?.methods?.every(
  (method) =>
    method.selected_destination_id &&
    method.groups?.every((group) => group.selected_option_id)
);

[].every(fn) evaluates to true, so an empty methods array satisfies the gate.

Fix: require the methods array to be non-empty before applying .every(...) (Array.isArray(methods) && methods.length > 0 && methods.every(...)). This matches the Python sample, whose complete_checkout blocks completion when methods is empty.

Category (Required)

• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)

Related Issues

None — newly identified bug in the Node.js sample server.

Checklist

• I have followed the Contributing Guide (Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable). — N/A, no documentation changes.
• My changes pass all local linting and formatting checks. (tsc --noEmit clean; prettier clean.)
• I have added tests that prove my fix is effective or that my feature works. (empty fulfillment methods array blocks completion.)
• New and existing unit tests pass locally with my changes. (Node.js suite: 29 passed.)
• (For Core/Capability) I have included/updated the relevant JSON schemas. — N/A, no schema changes.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk. — N/A, no schema changes.

Screenshots / Logs (if applicable)

Before/after — completing a checkout created with fulfillment: { methods: [] }:

Before (bug):

POST /checkout-sessions/{id}/complete -> 200   # order placed, no fulfillment

After (fix):

POST /checkout-sessions/{id}/complete -> 400   # "Fulfillment address and option must be selected"

Full Node.js suite:

# tests 29
# pass 29
# fail 0