fix(checkout-api): resolve checkout-race-condition

[Spkap]

Summary

Resolve checkout-race-condition for checkout-api with a validated ReplayX patch candidate.

Changed Files

• demo_app/src/inventory/reserve-stock.ts
• demo_app/src/checkout/submit-order.ts

Validation

• Concurrent checkout repro: exit 0
• Sequential checkout sanity: exit 0

Rollback

Revert the live inventory guard and concurrent checkout settlement handling.

[Copilot]

Pull request overview

This PR addresses the demo checkout oversell/race scenario by preventing inventory from going negative under concurrent checkouts and by treating “out of stock” as an expected outcome when running the concurrent repro harness.

Changes:

• Add a post-delay availability check in reserveStock to prevent overselling during concurrent reservations.
• Update the concurrent checkout scenario runner to use Promise.allSettled and only fail on non-OutOfStock errors.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
demo_app/src/inventory/reserve-stock.ts	Adds a post-delay inventory guard and aligns returned reservation metadata with the committed record state.
demo_app/src/checkout/submit-order.ts	Changes concurrent scenario execution to tolerate expected OutOfStock rejections while still surfacing unexpected failures.

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[Copilot]

runCheckoutRaceScenario is classifying expected failures by checking Error.message.startsWith("OutOfStock:"). This is brittle (message text changes/bundling can break it) and couples checkout flow to a string format. Prefer a structured signal (e.g., a dedicated OutOfStockError class, an exported type guard like isOutOfStockError(), or a stable error code property) and check that instead.

[Copilot]

The in-function comment above the delay still says the delay is an “intentional bug” that makes the pre-check stale, but the newly-added post-delay availability check changes the behavior (the stale pre-check is now mitigated). Please update that comment to reflect the current intent so future readers don’t think the race is still present by design.