fix(rest): emit discount totals[] entry as a negative amount

[vishkaty]

What

The Python Flower Shop server appends the applied discount to totals[] with a positive amount. Booting the server and creating a checkout for bouquet_roses (3500) with discounts.codes: ["10OFF"] returns:

"totals": [
  {"type": "subtotal", "amount": 3500},
  {"type": "discount", "amount": 350},
  {"type": "total", "amount": 3150}
]

Two problems with the +350:

1. Schema violation — total.json constrains discount/items_discount amounts with exclusiveMaximum: 0 (they must be strictly negative).
2. The receipt doesn't reconcile — 3500 + 350 ≠ 3150. Per discount.md ("Allocation Details"): "the applied[].amount describes the magnitude of the applied discount (always positive); the corresponding totals[] entry amount represents its signed effect on the receipt (negative for discounts)."

Fix

Negate only the totals[] entry (amount=-discount_amount). applied[].amount and the allocation stay positive, per the spec's magnitude-vs-signed-effect split. After the fix the same request returns {"type": "discount", "amount": -350} and subtotal + discount == total.

Tests

• New: test_discount_total_is_negative_and_receipt_reconciles — asserts the discount totals entry is strictly negative, applied[].amount stays positive, and the receipt reconciles.
• Corrected: test_discount_code_matches_case_insensitively asserted the old positive value ([100] → [-100]).
• Full rest/python/server suite passes.

[damaz91]

Thanks for the PR! The fix is correct and the new integration test verifies the behavior.

I have a couple of suggestions:

1. Regenerate Sample Output: Since the sign of the discount total has changed, the sample interaction log rest/python/client/flower_shop/sample_output/happy_path_dialog.md is now out of sync. Could you please regenerate it? You can do this by running the script rest/python/client/flower_shop/extract_json_dialog.sh (make sure your server is running or the script will start one) and then copying the generated file to the sample_output directory.

   Specifically, the totals section in the sample output should show a negative discount:

       {
         "type": "discount",
         "amount": -650
       }

2. Node.js Server Bug: I noticed that the Node.js implementation has the exact same bug. In rest/nodejs/src/api/checkout.ts around line 363:

             checkout.totals.push({ type: "discount", amount: discountAmount });

   It should be -discountAmount.

   If you'd like, you can fix it in this PR as well, or we can address it in a separate issue/PR.

Otherwise, this looks good to go.

[vishkaty]

Thanks for the review! Both addressed:

1. Sample output — updated the discount totals[] entries to -650 (3d3fb10). I kept it scoped to the discount lines: rather than a full extract_json_dialog.sh regen, which I found also sweeps in ~800 lines of unrelated modernization because the committed happy_path_dialog.md is still on the 2026-01-23 profile shape and predates the 2026-04-08 upgrade (fix(rest/python): update UCP version to 2026-04-08 #129) — single-transport services object, old spec URLs. Happy to bring the whole dialog current in a separate PR so this one stays focused on the discount fix; let me know if you'd prefer it here instead.

2. Node.js server — fixed the same bug (de0a7cb): totals[] discount is now -discountAmount, with applied[].amount left positive as the magnitude. Added a test driving recalculateTotals against an in-memory catalog (discount total negative, receipt reconciles, applied amount stays positive) — tsx --test is green (5/5), tsc clean.