This update prepares custom Python-based taxes to work more reliably in Point of Sale by making product tax data less restrictive and easier to extend. It also improves tax helper structure so localization and accounting modules can customize tax behavior without disrupting existing setups.

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This fixes an issue where Brazilian Pix QR codes could fail when the point of sale prepared an offline backup code without a payment amount. It helps ensure stores can keep using Pix payment QR codes reliably when the POS is offline.

QR code support was added to the POS [1]. A fallback QR code with a None amount is downloaded when the POS loads. This is used when the POS is offline.

This commit ensures we don't float_repr() None which results in:

TypeError: must be real number, not NoneType

[1] https://github.com/odoo/odoo/pull/148803