Ability to change subscription's collection method from platform

🔍 Problem Statement

Currently, changing a subscription’s collection method (for example, from charge_automatically to send_invoice) is supported through the Core API, but this functionality is not available in the platform “Update Subscription” interface which creates an operational gap.

💡 User Story

As a billing or customer support administrator,

I want to change a subscription’s collection method directly from the subscription update page,

So that I can switch customers between automatic charging and invoice-based billing without using external API calls or engineering support.

🎯 Definition of Done (DoD)

A feature is done when:

✔️ When updating a subscription from the platform:

1. Admin can view the current collection method.

2. Admin can select a different collection method.

3. Saving the subscription triggers the backend subscription update endpoint.

4. The validation rules that apply from core api should be applied on platform too. for example:

5. Switching to charge_automatically requires:

6. Valid default payment method

7. Chargeable customer state