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 adding a post-delay inventory availability guard and adjusting concurrent checkout handling so an expected OutOfStock outcome doesn’t fail the entire scenario run.

Changes:

• Add a post-delay record.available check in reserveStock to prevent inventory from going negative under concurrent reservations.
• Update reservation metadata to reflect the committed snapshotVersion.
• Switch concurrent checkout flow to Promise.allSettled, treating OutOfStock as a non-fatal outcome 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 guard and returns committed version/token to prevent negative inventory under concurrency.
demo_app/src/checkout/submit-order.ts	Uses Promise.allSettled to tolerate expected OutOfStock in concurrent mode while rethrowing other failures.

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[Copilot]

The comment about an "Intentional bug" (stale pre-check under concurrent checkout) is now outdated because a post-delay guard (record.available < quantity) was added. Please update or remove the comment so it reflects the current behavior/fix; otherwise it will mislead future readers when diagnosing race conditions here.

[Copilot]

runCheckoutRaceScenario is treating OutOfStock as a "soft" failure by inspecting Error.message.startsWith("OutOfStock:"). This is brittle (message text is not a stable contract) and duplicates the sentinel string across modules. Prefer a typed error (e.g., class OutOfStockError) or a stable discriminator (error code/name) exported from the inventory module and checked here.