Issue:
process_express_checkout assigns partner_id directly on an anonymous cart (removing pricelist_id from the fields to recompute, to keep the pricelist stable), so the address update is never called. The order fiscal_position_id still refreshes via its stored compute, but the line tax_id stays with the previous mapping: _compute_tax_id does not depend on fiscal_position_id, so it is never re-triggered. The invoice is then posted with the wrong VAT.

Recomputing the taxes silently on the backend is not enough: the customer already approved a total in the native express payment sheet (Apple Pay, Google Pay or Link), and we never update that amount. They would then be charged a different, higher total than the one they accepted.

Fix:
When the billing address resolves to a different fiscal position than the one used to quote the displayed amount, recompute the taxes and abort the payment: the endpoint returns False and sets a shop_warning, and the Stripe express checkout form reloads the page so the customer sees (and re-accepts) the refreshed prices before retrying. _recompute_taxes is wrapped in a try/except UserError to avoid blocking the flow if an external tax computation (e.g. Avatax) fails.

Steps to reproduce:
- Configure the company with two auto-applied fiscal positions, e.g. a domestic one (Luxembourg, 3% sale tax) and an "Extra-Community" one that maps the domestic tax to 0% EC for non-EU countries.
- Configure Stripe with an express payment method (Apple Pay, Google Pay or Link).
- Log out and browse the shop with a geoip pointing to a non-EU country so the cart picks up the "Extra-Community" fiscal position.
- Add a product to the cart.
- Observe: the cart line has the 0% EC tax.
- Trigger the express payment button and pay using a billing address located in the domestic country (Luxembourg).
- Observe on the confirmed order: fiscal_position_id is the domestic one but the line tax_id still holds the 0% EC tax. The invoice is posted with 0% VAT instead of the expected 3%.

opw-6213195

Description of the issue/feature this PR addresses:

Current behavior before PR:

Desired behavior after PR is merged:




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