**Issue**
When buying products in the Brazilian localization, certain errors in external tax calculation were not properly caught by the frontend. This caused the checkout to hang indefinitely with infinite loading. Examples include missing NCM codes or IAP service failures due to invalid addresses.

**Steps to Reproduce**
1. Install Brazilian localizations (l10n_br, l10n_br_avatax, l10n_br_edi).
2. Configure Avatax Transfer API credentials (API ID and Key).
3. Create a website with a Brazilian company.
4. Add a product to the cart and proceed to checkout.
5. Choose a delivery method and observe that the UI gets stuck loading.

**Root Cause**
The `_order_summary_values` method in `website_sale_external_tax` called `_get_and_set_external_taxes_on_eligible_records()`, which could raise exceptions (e.g., IAPServerError). These exceptions were not handled, so they propagated to the frontend as generic RPC errors. The frontend has no built-in mechanism to display these exceptions as user-friendly messages, resulting in infinite loading.

**Fix**
Wrap the external tax calculation in `_order_summary_values` and catch `UserError`. 
Instead of letting the exception propagate as a generic RPC error, attach the error 
message to the result dictionary under `external_tax_error`. This prevents the 
frontend from hanging while still making the underlying problem visible in the 
next checkout step, where validation errors are properly handled and shown to the user.

Opw-5052078

Forward-Port-Of: odoo/enterprise#96213
Forward-Port-Of: odoo/enterprise#95045

**Issue**
When buying products in the Brazilian localization, certain errors in external tax calculation were not properly caught by the frontend. This caused the checkout to hang indefinitely with infinite loading. Examples include missing NCM codes or IAP service failures due to invalid addresses.

**Steps to Reproduce**
1. Install Brazilian localizations (l10n_br, l10n_br_avatax, l10n_br_edi).
2. Configure Avatax Transfer API credentials (API ID and Key).
3. Create a website with a Brazilian company.
4. Add a product to the cart and proceed to checkout.
5. Choose a delivery method and observe that the UI gets stuck loading.

**Root Cause**
The `_order_summary_values` method in `website_sale_external_tax` called `_get_and_set_external_taxes_on_eligible_records()`, which could raise exceptions (e.g., IAPServerError). These exceptions were not handled, so they propagated to the frontend as generic RPC errors. The frontend has no built-in mechanism to display these exceptions as user-friendly messages, resulting in infinite loading.

**Fix**
Wrap the external tax calculation in `_order_summary_values` and catch `UserError`. 
Instead of letting the exception propagate as a generic RPC error, attach the error 
message to the result dictionary under `external_tax_error`. This prevents the 
frontend from hanging while still making the underlying problem visible in the 
next checkout step, where validation errors are properly handled and shown to the user.

Opw-5052078

Forward-Port-Of: odoo/enterprise#96213
Forward-Port-Of: odoo/enterprise#95045