fix(next): reuse subscription item fetch logic

[StaberindeZA]

Because

• Reuse function to retrieve subscription item from subscription.

This pull request

• Move retrieveSubscriptionItem into util function and reuse in invoice.manager.
• Extract logic to fetch upcoming upgrade invoice into previewUpcomingUpgrade method.
• Rewrite previewUpcomingForUpgrade to more accurately reflect its purpose.

Issue that this pull request solves

Closes: (issue number)

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• If applicable, I have modified or added tests which pass locally.
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• I have verified that my changes render correctly in RTL (if appropriate).

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