Support per-checkout-link/session tax_behavior override

[stilla]

Currently tax_behavior (inclusive/exclusive) only resolves from ProductPrice.tax_behavior falling back to Organization.default_tax_behavior. There's no way to override it per checkout link or checkout session.

Use case: merchant's org default is exclusive, but wants a specific checkout link (e.g. for an upsell or A/B test) to render inclusive of VAT, without creating a separate price/product.

Add a tax_behavior override field to CheckoutLinkCreate/CheckoutLinkUpdate and/or CheckoutCreate/CheckoutUpdate, and thread it into CheckoutService._update_checkout_tax (checkout/service.py) with precedence over the price-level and org-level defaults.

Related: #4788 (original tax_behavior design), #13470 (tax_behavior checkout calculation edge case).

_{Sent by @allison-polar from VAT inclusive checkout link implementation.}

Plain ticket

[pieterbeulque]

@frankie567 - Does this fit the API philosophy? It matches some of the other overrides we allow, I guess?

[frankie567]

Yes, I guess it matches. The main question is: is it such an important use-case to support?

Reading at the ticket, it seems like what the merchant wants is the setting at product level, which we already support. So I don't think it's worth to add this option at CheckoutLink/Checkout level.