### Description of the feature this PR addresses:
**Specific tax on withholdings for incapacity to work created to correctly reflect the amounts derived in Form 190 of the AEAT.**

A new tax called "**100% WHI monies**" has been created with the aim of correctly reflecting the income derived from incapacity for work in the AEAT's Model 190.

### Current behavior before PR:

Currently, Odoo does not allow automatic differentiation between monetary payments derived from work incapacity and those not derived from it.
This creates a problem when generating Model 190, as the Spanish Tax Agency (AEAT) requires both types of payments to be reported separately (according to the codes and subcodes established in the model).

**In practice, this separation is necessary because:**

- Income derived from incapacity for work (e.g., sick leave) is considered employment income with differentiated tax treatment.
- Withholdings made on these amounts must be declared in specific sections of Form 190.
- If they are not separated correctly, the totals on the form may be incorrect or incomplete, affecting the accuracy of the tax information submitted.

### Desired behavior after PR is merged:

A new tax has been added to represent “100% Withholding income tax (workers)”, allowing Odoo to:
- Correctly identify and calculate the related amounts.
- Report them separately in the AEAT Model 190.
- A new tax group has also been created to distinguish these withholdings from regular ones.

### Benefits:

- It allows you to correctly generate Form 190, separating income derived from and not derived from work disability.
- It facilitates the accounting and tax traceability of this type of income.
- It ensures consistency with the information structure required by the Spanish Tax Agency (AEAT), avoiding errors in the annual filing.
- It improves the flexibility of the Odoo tax system, adapting it to actual tax needs.

### Technical summary:

- A **new specific tax** has been created for withholdings associated with incapacity for work benefits.
- This tax **allows the amounts corresponding to these benefits to be identified, accounted for, and reported independently.**
- In addition, a **new tax group "Whitholdings IRPF Workers" has been added that groups these withholdings together**, allowing them to be clearly distinguished from other withholdings on earned income.

The related issue: https://github.com/OCA/l10n-spain/issues/4416

### Follow-up suggestion: adjust Odoo’s base tax “15% WHI monies”

**Proposal:** Change the base tax “15% WHI monies” to 100% and move it to the new “Whitholdings IRPF Workers” tax group.

**Why:** The current 15% rate is not aligned with how AEAT Model 190 requires amounts to be computed: the base should reflect 100% of the earnings derived from temporary incapacity, with the withholding rate applied on top of that base. Keeping 15% leads to misclassification and wrong totals in the "_percepciones_" and "_retenciones_" sections.

These are variable withholdings, not fixed taxes — each worker’s rate can change monthly depending on their income and number of days worked. Therefore, having a fixed 15% rate is conceptually incorrect and causes misclassified amounts and inaccurate totals, forcing users to adjust values manually after payroll.

Ideally, this tax should not have any fixed percentage defined. Since Odoo currently requires a rate, setting the base to 100% is a pragmatic workaround that ensures more accurate and flexible withholding calculations.

**Outcome:**

- Correct calculation and reporting of **_percepciones derivadas de incapacidad laboral_**.
- Clean separation from other withholdings via the new tax group.
- Consistency between accounting, payroll, and the 190 export.

Would you accept this change?


@moduon @rafaelbn @EmilioPascual @chklop @jco-odoo MT-11667 

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Forward-Port-Of: odoo/odoo#232515