Steps to reproduce:
---------------------------
1. Install the `project_account_budget` and `account_accountant` modules.
2. Create a new project and add an Analytic Account for it from the settings page
3. Open the Project Kanban, click the three dots on the project card, and select Project's Updates.
4. Click Add Budget button and open the budget wizard.
5. Add a budget line in the wizard with a planned amount expressed as a negative value for an expense (for example: -10000).
6. Create a Vendor Bill using the same analytic account with an amount of 1000.
5. Confirm the bill.
6. Go back to Project's Updates and click New button to view the budget summary.

Observation:
---------------------------
The budget summary displays incorrect signs and percentages in Activities summary, for example:
```
-10.0% (-1,000.00) of the -10,000.00 budget has been spent. 110.0% (-11,000.00) of the budget is remaining.
```
This incorrectly shows -10% spent and 110% remaining instead of 10% spent and 90% remaining (-9,000).

Issue:
---------------------------
The project cost (already negative) was negated again when computing the spent amount in
https://github.com/odoo/enterprise/blob/ac3f333d97eda5c86a0813490ac6204d4ec5721f/project_account_budget/models/project_update.py#L16 Double-negating the cost makes it positive, which then gets added to the expense budget instead of reducing it, producing inverted percentages and signs.

Solution:
---------------------------
For expense budgets (negative budgets), do not apply an extra negative sign when calculating the project cost so the spent, remaining, and percentage values are computed correctly.

After the fix:
```
10.0% ($ 1,000.00) of the $ -10,000.00 budget has been spent. 90.0% ($ -9,000.00) of the budget is remaining.
```

opw-5357854

Forward-Port-Of: odoo/enterprise#109638
Forward-Port-Of: odoo/enterprise#102126