Allow finalizing an order if fully covered by store credits

[rainerdema]

Summary

Fixes #307

This temporary fix enables the finalization of orders entirely covered by store credits or adjustments, but it can't be a permanent solution.

The main "issue" relates to Stirpe, which does not allow a 0$ charge because the minimum amount needed to perform the operation isn't met: https://stripe.com/docs/currencies#minimum-and-maximum-charge-amounts

Various alternative strategies could be followed:

• One option could be to hide the payment method from the frontend interface.
• Map the minimum amounts of all the mentioned currencies and perform a payment with that amount in case of orders fully covered by store credits/adjustments
• Another possibility is to integrate a structure into the Spree::PaymentMethod that can prevent a payment method from appearing among the choices available for an order in available_payment_methods.

ℹ️ Touching the payment_required? method on Spree::Order is something I didn't consider as it would prevent the processing of payments and possibly even the applied store credits, but I haven't looked into that so much, so let me know.

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Screenshots

[rainerdema]

@solidusio/core-team thoughts on https://github.com/solidusio/solidus_stripe/pull/310#issue-1725969255?

[codecov[bot]]

Codecov Report

Merging #310 (c970554) into main (b7dfc00) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #310   +/-   ##
=======================================
  Coverage   99.64%   99.64%           
=======================================
  Files          31       31           
  Lines         563      564    +1     
=======================================
+ Hits          561      562    +1     
  Misses          2        2           

Impacted Files	Coverage Δ
...lates/app/views/checkouts/payment/_stripe.html.erb	100.00% <100.00%> (ø)

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[waiting-for-dev]

I think, ideally, our business logic should be able to move forward when there's no required money. If that's not possible, then we should tackle it at the view layer, but I wouldn't enforce a minimum amount.

[rainerdema]

This will be addressed with: Preventing available payment method rendering for orders fully covered by Store Credits #345