[REF] contract - pass self instead of relying on @api.model

[anthonissen-a]

Replaces @api.model methods with instance methods across contract methods to improve extensibility and allow overrides to access record data in self.

Method parameters are now optional to ensure full backward compatibility.

[OCA-git-bot]

Hi @sbejaoui, @florian-dacosta,
some modules you are maintaining are being modified, check this out!

[qgroulard]

Great changes 👍

This is backward compatible and it highly improves the extensibility of get_next_invoice_date and get_next_period_date_end for other OCA modules and custom code.

[tobiaszehntner]

LGTM

[OCA-git-bot]

This PR has the approved label and has been created more than 5 days ago. It should therefore be ready to merge by a maintainer (or a PSC member if the concerned addon has no declared maintainer). 🤖

[sbejaoui]

let see what @pedrobaeza thinks obout this

[pedrobaeza]

I don't think it's a good idea, as this is mixing both approaches: if you use self, then you should use it for all, but if there's going to be options for still passing arguments, then keep it @api.model, and you will get a predictive behavior, as having some arguments that if omitted, are taken from self implies some kind of "black magic".