[Split PE] Implement Split PE for cards in the block checkout

[a-danae]

Acceptance criteria

• When multiple payment method types are enabled, they must appear as different payment gateways in the classic checkout, as it happens when PE is disabled. Design xVAk98kbRKUByVTrhPc98g-fi-342_8806
• Payment processing must continue to work for cards
• Changes must be covered by tests
• e2e tests must be adjusted to at least maintain the same coverage they had before split PE

Notes

This might be handled in #2784. Confirm before grabbing this issue.

[james-allan]

This has been handled in #2784.

On the latest add/deferred-intent working branch, Cards with split have been implemented exclusively.

No other alternative payment method has been implemented yet.

The way I implemented it for #2784, it loops over all the payment methods IDs (ref) and attempts to register the Payment method with the Blocks API (ref).

The reason this doesn't work to implement split for all APMs is because those payment methods haven't been registered via the $payment_method_registry (see this guide) and so it doesn't load. That's the next step as part of #2790.

Given cards have already been implemented with Split UPE and they work to take card payments I'm going to close this as fixed in #2784.