Currently, the customer, who did not perform the operation, is set 
as the author of the refund, capture, and void transaction messages.

**Steps to reproduce:**
- Install the `sale_management` module and enable the `demo` payment provider.
- Create a new quotation and from the `gear` icon, select `Mark Quotation as Sent`.
- From the `gear` icon, choose `Generate a Payment Link`, and copy the link.
- Open the link in an incognito window, and complete the payment.
- Return to the original tab and open the payment transaction from the chatter.
- Click `Refund` > `Refund` and observe the `author` of the messages in the chatter.

**Observation:**
The refund messages entry shows the customer as the author instead of the 
logged-in user who executed the refund.

**Root Cause:**
At [1] and [2], when the logged-in user is not a superuser, the code assigns 
the customer (`self.partner_id`) as the author, even during `refund`, `capture`, 
and `void transactions`, resulting in an incorrect message author.

**Fix:**
This commit ensures that during refund, capture, and void transactions, 
the message author is set to the logged-in user instead of the customer.

[1]:
https://github.com/odoo/odoo/blob/bbf3bd7b0e1d3e015350f9c959fad056384e2318/addons/account_payment/models/payment_transaction.py#L223

[2]:
https://github.com/odoo/odoo/blob/5cf325dfff1ffbc1c4b4d0bcfc44ac9f55ea7c51/addons/sale/models/payment_transaction.py#L110

**Before:**
<img width="1910" height="624" alt="5379376_before" src="https://github.com/user-attachments/assets/decdd620-d741-488e-9dec-e5c8c9bafc48" />

**After:**
<img width="1917" height="622" alt="5379376_after" src="https://github.com/user-attachments/assets/5f38f3c3-fd2c-489e-afc9-d7a356fde23a" />

opw-5379376