fix(rest/nodejs): scope idempotency hashes to checkout operations

[XiaolongZhang-TT]

Description

The Node.js sample keyed idempotency records only by the Idempotency-Key header and fingerprinted the request body alone, so the same key reused against a different checkout (or a different operation) replayed the first response instead of conflicting. This was most severe for cancel, whose hash was always the empty body, so any two cancels sharing a key were treated as identical regardless of target.

Repro: create two checkouts A and B, then cancel both with the same Idempotency-Key → the second cancel returns 200 with A's canceled checkout (silent replay) instead of conflicting.

Root cause — rest/nodejs/src/api/checkout.ts, computeHash(data): the fingerprint was the body only (cancel: {}), with no operation or target checkout.

Fix: include the operation and, for update/complete/cancel, the target checkout id in the fingerprint — computeHash(operation, data, resourceId?). This mirrors the Python sample (#166): create is keyed by body, update/complete/cancel additionally by the path checkout id, so a key can neither cross checkouts nor cross operations.

Category (Required)

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

Related Issues

None — parity with the Python sample's idempotency scoping landed in #166.

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. (an idempotency key is scoped to the checkout (cancel) and …(update) — both fail before the fix, pass after.)
• New and existing unit tests pass locally with my changes. (Node.js suite: 118 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 — reusing one Idempotency-Key against two different checkouts:

Before (bug):

cancel A (key=k) -> 200
cancel B (key=k) -> 200   # replayed A's canceled checkout

After (fix):

cancel A (key=k) -> 200
cancel B (key=k) -> 409   # Idempotency key reused with different parameters

Full Node.js suite:

# tests 118
# pass 118
# fail 0