fix(rest/nodejs): express discount allocation path as a JSONPath

[XiaolongZhang-TT]

Description

discount.json types allocation.path as a JSONPath (RFC 9535) pointing at the allocation target (e.g. "$.line_items[0]"). The Node.js sample emitted the bare label "subtotal" for every applied discount's allocation, which a platform cannot resolve as a path. The Python sample already emits "$.totals[?(@.type=='subtotal')]".

Root cause — rest/nodejs/src/api/checkout.ts, recalculateTotals: all three discount code branches (10OFF, WELCOME20, FIXED500) built allocations: [{ path: "subtotal", amount }].

Fix: emit "$.totals[?(@.type=='subtotal')]" instead — the same JSONPath the Python reference uses. totals is an array of { type, amount } objects, so the filter expression is the correct way to target the subtotal entry (a dot-path would be wrong). Allocation amounts are unchanged.

Category (Required)

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

Related Issues

None — newly identified spec-compliance gap 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. (an applied discount's allocation path is a JSONPath — fails on the old "subtotal" value.)
• New and existing unit tests pass locally with my changes. (Node.js suite: 30 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 — an applied discount's allocations[0].path:

Before (bug):

"allocations": [{ "path": "subtotal", "amount": 100 }]

After (fix):

"allocations": [{ "path": "$.totals[?(@.type=='subtotal')]", "amount": 100 }]

Full Node.js suite:

# tests 30
# pass 30
# fail 0