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 checkout-race-condition incident in the demo app’s checkout flow by preventing concurrent reservations from overselling inventory and by making the concurrent repro scenario tolerate expected “out of stock” outcomes.

Changes:

• Add a post-delay, pre-decrement availability guard in reserveStock to prevent inventory from going negative under concurrent checkout attempts.
• Return reservation metadata (token + snapshotVersion) from the committed inventory record rather than a potentially stale snapshot.
• Update the concurrent checkout repro to use Promise.allSettled, treating OutOfStock as an expected rejection 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 second availability check after the async delay and returns commit-time snapshotVersion/token to avoid stale results.
demo_app/src/checkout/submit-order.ts	Switches concurrent runner to allSettled and filters expected OutOfStock rejections so the repro can succeed without masking hard failures.

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