The`SP` (Split Payment) taxes have incorrect configurations:
- **Tax Grid Tags on `SP` Taxes**: The ve38 grid tag was incorrectly assigned to the tax line instead of the base line.
- **`VE38` VAT Report Formula**: In Tax Report > VAT Report, the line `VE38 - Transactions with parties referred to in Article 17-ter` was using a negative formula `-ve38` and should be `ve38`
- **Tax Group Invoice Label**: Updated the default label on invoices for the SP tax group to display 22% SP instead of 22%.

Steps to reproduce:
- Install `account` and `l10n_it`
- Switch to IT company
- Go to taxes and filter for `SP` taxes
- The tax grid is wrong for the negative taxes since `ve38` should be in the base line instead of in the tax line
- Go in the Tax report > VE VAT Report
- The line `VE38 - Transactions with parties referred to in Article 17-ter` has a negative formula `-ve38`, should be positive `ve38`
- The label on invoices of the tax group should be `4% SP`, `5% SP`, `10% SP` not `4%` etc.

References:
https://www.informazionefiscale.it/IMG/pdf/dichiarazione_modello_iva_2026_agenzia_delle_entrate.pdf
<img width="1413" height="141" alt="immagine" src="https://github.com/user-attachments/assets/7fa15859-beaa-4345-bf81-fedc3f0af2fa" />
https://fiscomania.com/quadro-ve-della-dichiarazione-iva/
<img width="705" height="154" alt="immagine" src="https://github.com/user-attachments/assets/4f23b407-0e4d-4104-9695-f9891ccfd2f2" />

Ticket [link](https://www.odoo.com/odoo/project.task/6543520)
opw-6543520

Forward-Port-Of: odoo/odoo#287238

### Issue before this commit:
When generating a credit note for an invoice already accepted in the Polish KSeF system, the exported XML incorrectly includes the <NrKSeFN>1</NrKSeFN> tag. This tag falsely indicates that the original invoice was issued outside of KSeF, and the official KSeF number of the corrected invoice is entirely omitted from the file.

### Steps to reproduce the issue:
1. Download Accounting and l10n_pl
2. Go to Settings > Polish Localization > Insert certificate
3. Go to Invoices > Create an invoice > Send it to KSeF
4. Create a credit note for that invoice, reverse it and send it to KSeF
5. Tag <NrKSeFN>1</NrKSeFN> should not be there

### Cause of the issue:
The tag <NrKSeFN>1</NrKSeFN> is emitted in every case without any rule handling it.

### Reason to introduce the fix:
To comply with the official FA(3) logical structure rules. According to the specifications, if the corrected invoice was issued and accepted in KSeF, the XML must include the <NrKSeF>1</NrKSeF> flag and additionally provide the original KSeF number in the <NrKSeFFaKorygowanej> field. Otherwise, if the original invoice was issued outside of KSeF, the system must enter "1" in the <NrKSeFN> field and strictly omit the <NrKSeF> and <NrKSeFFaKorygowanej> fields.

<img width="866" height="981" alt="image" src="https://github.com/user-attachments/assets/53b42f9f-aaab-4642-be38-2a5abc1d8539" />

opw-6541941

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Forward-Port-Of: odoo/odoo#288286