When importing a WinBooks zip, contacts sharing the same number might fail VAT validation if distinct contact data is merged incorrectly.

Steps to reproduce:
- Create a database with the Belgian localization installed
- Install the account_winbooks_import module
- Navigate to Accounting > Configuration > Settings > Initial Setup > Import
- Import the WinBooks zip

Issue:

The import fails with the following error:
The VAT number [AAAAAA] for partner [BBBBBB] does not seem to be valid. Note: the expected format is BExxxxxxx.

Analysis:
In WinBooks, contacts can be of type Supplier or Customer, and it is possible for a supplier and a customer to share the same number. During the import, the system will merge contacts if found with the same number.

In the specific case, the VAT number from one contact without country is combined with the country information from another contact, resulting in a VAT validation error.
This change prevents the merger of partner data if their VAT numbers are explicitly different.

opw-6251772

Forward-Port-Of: odoo/enterprise#119579

Issue:
When a shopper pays for an online order entirely using a reward (e.g. a discount code covering 100% of the total), the order total is zero. With automatic invoicing enabled, an invoice with an amount of 0 is created. But, the 0 invoice is never emailed to the customer. Whereas, for orders where the total is more than 0, an invoice is created, then emailed to the customer. The customer should be emailed the invoice, even if its amount is 0.

Steps to reproduce:
1. Enable automatic invoicing.
2. Create a code for a 100% discount.
3. As a shopper, add a product to cart on the website.
4. While checking out, apply the 100% discount code to the order.
5. Complete the checkout.
6. Confirm that an invoice was created and posted, but was not emailed to the customer.

Explanation:
Normally, order confirmation and invoicing are handled by the `_post_process` method on the `payment.transaction` model. With automatic invoicing enabled, `_post_process` confirms the sale order, creates the invoice, and sends the invoice via `_send_invoice` (another method on the `payment.transaction` model). If `sale.async_emails` is enabled, `_post_process` will trigger a cron that invokes `_send_invoice` instead of invoking it directly.

When an order is fully covered by a reward, there's nothing to pay. In this case, no payment.transaction record is ever created, and `_post_process` never runs. Instead, the order is confirmed through the `_validate_order` method on the `sale.order` model. The `sale_loyalty` module extends `_validate_order` so that, with automatic invoicing enabled, it will create and post an invoice for zero-amount orders. But, nothing in this path ever calls `_send_invoice` or an equivalent. So, the invoice is created and posted but never sent.

Solution:
This adds logic for sending invoices to the extension of `_validate_order` in the `sale_loyalty` module. We mirror the logic used in `_send_invoice` in the `payment.transaction` model.

Notes:
There is duplicated code from `_send_invoice` in this fix. That is because `_send_invoice`, a method on the `payment.transaction` model, can't be used in this flow. A fix that avoids code duplication would require serious refactoring.

This will never trigger a cron to send the invoice, even if `sale.async_emails` is enabled. That is because the cron invokes `_send_invoice`. Since fully reward-covered orders are probably not common, any performance benefits of using a cron are probably not significant. But, making a new cron to be used in this case is also an option.

opw-6363334

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Forward-Port-Of: odoo/odoo#277678
Forward-Port-Of: odoo/odoo#275190