When we create new payments for bills and refunds, we offer the possibility to merge inbound and outbound payments together if they are from the same provider and the bills all reference the same recipient account.

Depending on the balance of the resulting payment, we assign an adequate inbound or outbound bank account as the recipient.

The `partner_bank_id` can be assigned through different processes:

- If the wizard has only one batch:

The wizard is editable and the user can select a bank account from the computed `available_partner_bank_ids`.

- If there are multiple batches:

Odoo assigns a `partner_bank_id` in `_create_payment_vals_from_batch()`. For an outbound payment, it uses the batch['payment_values']['partner_bank_id'].

The problem is that this value is not updated after a merge of payments. If the base line being merged on is a refund, but the result is an outbound payment, then the `partner_bank_id` should be changed accordingly.

I decided to include the changes of my previous PR targeting v18 and fixing the grouping of payments, even though it was deemed unnecessary for v17, because it felt weird not to considering how close these fixes are.

-Previous PR : [242863](https://github.com/odoo/odoo/pull/242863)

However, I can remove these changes or re-target this PR to v18. I am not sure what would be best here.

Ticket: opw-5401372

Forward-Port-Of: odoo/odoo#246558

**Steps to reproduce:**
* Install the **Accounting** module.
* Create a product category hierarchy (grandparent → parent → child).
* Set an **income account** on the grandparent category.
* Set an **expense account** on the child category.
* Leave the parent category without any accounts.
* Create a product assigned to the child category.
* Create a customer invoice using this product.

**Observed behavior:**
* The invoice line uses the **default income account** from settings instead of the grandparent category’s income account.
* The category hierarchy is not checked to retrieve parent accounts.

**Expected behavior:**
* If no account is defined on a category, Odoo should traverse the **parent categories** to find one.
* Only if no account exists in the hierarchy should it fall back to journal or default accounts.

**Cause:**
* Account lookup only checked the **immediate category**.
* Parent categories were not considered.

**Fix:**
* Traverse the product category hierarchy when searching for accounts.
* Use the first account found in the parent chain.

opw-5869592

Forward-Port-Of: odoo/odoo#248389