fix: logic: 'following calendar months' now allows yearly billing cycles

[0spinboson]

For my association I wanted to be able to create yearly memberships tied to open-ended subscriptions that follow the calendar year. This was impossible because of validation errors and missing logic.

In the current codebase, there is a check for the existence of a subscription end date when "follow calendar months" is enabled. It complains about the absence of a subscription end date when you create a subscription on a yearly billing cycle. This exception seems unintentional and I cannot think of a valid use case where you'd want to require a predefined subscription end date with yearly subscriptions but not monthly/quarterly. As such, I am submitting this fix (which otherwise leaves the validation check in place).

The second and third commit expand the get_current_invoice_end method invoicing logic to work with yearly subscriptions as well as amending the option description in the json file.

[ruthra-kumar]

bump

[stale[bot]]

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[0spinboson]

whoops, premature push, sorry.

[0spinboson]

oh, this isn't better. sorry number 2

[0spinboson]

PR hopefully superseded by https://github.com/frappe/erpnext/pull/48770

[mergify[bot]]

⚠️ The sha of the head commit of this PR conflicts with #48770. Mergify cannot evaluate rules on this PR. ⚠️