fix(payments-next): UpdateStripeProcessingCartError: Stripe checkout cart state not changed to processing: Cart version mismatch

[elizabeth-ilina]

Because

• When a customer updates their tax location on checkout, sees that tax location has been updated successfully message, but CheckoutForm may still be re-rendering with the older cart version. If the customer clicks Subscribe before the new cart version finishes re-rendering, a cart version mismatch occurs and an UpdateStripeProcessingCartError is thrown.

This pull request

• Adds a shared client state between SelectTaxLocation and CheckoutForm, tracking whether a cart-mutating action is in progress.
• Disables the Subscribe button and defers the "Your location has been updated" banner until the client's cart version advances.

Issue that this pull request solves

Closes: #PAY-3654

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
• I have manually reviewed all AI generated code.

How to review (Optional)

STRs:

1. Make an account, be on checkout
2. Fill in card details, don't click on subscribe yet
3. Update postal code, click Save, and very quickly press subscribe after you see "Your location has been updated"

Before:

Screen.Recording.2026-05-22.at.11.39.27.AM.mov

Admin Panel, very similar carts to affected prod customers:

After:

Screen.Recording.2026-05-22.at.11.45.37.AM.mov

• Key files/areas to focus on:
• Suggested review order:
• Risky or complex parts:

Screenshots (Optional)

Please attach the screenshots of the changes made in case of change in user interface.

Other information (Optional)

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