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 demo app’s checkout-race-condition by preventing overselling during stock reservation and treating expected OutOfStock failures as non-fatal during concurrent checkout repro runs.

Changes:

• Add a post-delay inventory availability guard in reserveStock to prevent committing negative inventory under concurrent requests.
• Adjust reservation token/version to reflect the committed inventory record version.
• Update concurrent checkout scenario runner to use Promise.allSettled, ignore OutOfStock rejections, and only fail on unexpected 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 availability check and aligns returned token/version with committed record state.
demo_app/src/checkout/submit-order.ts	Switches concurrent mode to allSettled, filters expected OutOfStock failures, and returns only successful results.

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[Copilot]

The comment above this delay says the pre-check becomes stale under concurrent checkouts, but the new post-delay record.available guard makes this no longer an intentional bug. Please update/remove the comment so it accurately describes the current behavior (e.g., that we re-check after the delay to avoid overselling).

[Copilot]

This logic treats "OutOfStock" as a non-fatal rejection by matching on Error.message.startsWith("OutOfStock:"), which is brittle and can easily break if the message changes or the error is thrown from a different realm/type. Consider using a dedicated OutOfStockError class (or an error.code/name field) exported from the inventory layer and checking that instead of parsing the message string.