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

Resolves the checkout-race-condition in checkout-api by preventing overselling during concurrent inventory reservations and adjusting concurrent checkout handling to tolerate expected out-of-stock failures.

Changes:

• Add a commit-time (post-delay) inventory availability guard in reserveStock to prevent negative inventory under concurrency.
• Make concurrent checkout use Promise.allSettled and treat OutOfStock rejections as expected while still surfacing unexpected failures.

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 availability check and aligns returned reservation metadata with the committed record state.
demo_app/src/checkout/submit-order.ts	Switches concurrent execution to allSettled, ignoring expected OutOfStock failures while propagating other errors.

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[Copilot]

Classifying an expected failure by checking error.message.startsWith("OutOfStock:") is brittle (message text changes will alter control flow). Prefer a dedicated OutOfStockError (or an error.code field) thrown from reserveStock, and filter using instanceof/code instead of string parsing.

[Copilot]

Now that a post-delay availability check is added (and the concurrent negative-inventory path should be prevented), the nearby comment describing this delay as an "Intentional bug" is misleading. Update the comment to reflect the current behavior (e.g., that the delay simulates latency but the commit-time check prevents oversell).