[IMP] subscription_oca:set invoice currency to subscription pricelist and ensure company context in create_invoice

[stferraro]

In multi-company and multi-currency setups, invoices generated from subscriptions may not consistently enforce:

The subscription pricelist currency on the created invoice.

The subscription company context during invoice creation.
This can lead to inconsistent invoice values or cross-company context issues in downstream implementations.

Describe the solution you'd like
Implement the following in subscription_oca:

1. Ensure invoice values explicitly set currency_id from subscription.pricelist_id.currency_id when preparing account.move values.

2. Ensure create_invoice runs under subscription.company_id context (with_company) before creating the invoice.

Also add unit tests to validate:

1. Invoice currency matches subscription pricelist currency.

2. Invoice company matches subscription company in multi-company scenarios.

Describe alternatives you've considered

1. Keep this behavior in downstream custom modules only.

2. Override invoice creation in each deployment that needs strict multi-company/multi-currency behavior.

These alternatives duplicate logic across projects and increase maintenance cost compared to handling it upstream.

Additional context
I can submit a PR for 19.0 including implementation and tests.

[stferraro]

Hi, @sbidoul @CristianoMafraJunior @thib-d @yvaucher check this functionality added to subscription oca and approve please