This commit does not bring native support for EPD on credit notes, only makes custom support a little easier and cleaner.

It is quite easy to support EPD (early payment discounts) on refunds by extending
- `_early_payment_discount_move_types`
- `_is_eligible_for_early_payment_discount`

However, this approach breaks when it reaches `inverse_tax_rep` in `_get_invoice_counterpart_amls_for_early_payment_discount_per_payment_term_line`, which assumes tax repartition lines with `document_type == 'invoice'` and raises when called on `tax_rep` lines of 'refund' type instead.

This commit fixes that by selecting source and target repartition lines according to the `tax_rep`'s document type, which ensures:
- the `.index()` no longer raises a `ValueError`, as `tax_rep` is now looked up in the matching set (`refund_` for refunds, `invoice_` otherwise)
- `inverse_tax_rep` returns the corresponding line in the opposite set, preserving the original invoice->refund mapping while adding the refund->invoice one

Since `inverse_tax_rep` is a closure, downstream modules cannot patch it without copying the whole ~170-line method. Making it symmetric here lets custom EPD-on-refund support work without that duplication.

task-[6265601](https://www.odoo.com/odoo/all-tasks/6265601)

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