feat: intentionally break checkout flow for QA benchmark

[wenhaog7]

Summary

This PR intentionally introduces a regression in the cart flow for QA-agent benchmarking.

After this change, clicking Add to cart (or calling addCartItems) does not add items to the cart.
The mutation returns the existing cart state, so the UI appears to do nothing.

Why

We need a deterministic “bad commit” to validate that our UI testing agents can detect real user-facing regressions from code diffs.

What Changed

• Updated packages/api-plugin-carts/src/mutations/addCartItems.js
• Short-circuited add-to-cart behavior so cart contents are not updated
• Existing cart is returned without item additions

Expected User Impact

• Product page: Add to cart click has no effect
• Cart badge/count does not increase
• Cart content remains unchanged

Validation

1. Start Reaction API + storefront locally
2. Open storefront and click Add to cart
3. Confirm cart stays unchanged
4. Run ui-testing-agent diff-analyze against this commit to verify agent catches regression

Notes

• This PR is for benchmark/demo only.
• DO NOT MERGE into production branches.

[changeset-bot]

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Latest commit: b385eef

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