Close payment-gateway render loop and repair gift-card/coupon submit

[acasazza]

Two independent render/no-op bugs, both surfacing in mfe-checkout.

PaymentGateway — single-method mismatched-amounts render loop (#803)

The payment-source reconcile effect is reworked around a useEffectEvent, so its dependency array is a deliberate trigger set of stable ids and scalars rather than whole objects. Customer-source refetches that mint new order / paymentSource identities no longer re-fire it.

The single-payment-method branch now recreates a source when it is mismatched, not only when absent or errored, and refetches customer sources only after a real (re)create — so a no-op pass cannot re-arm the loop.

The three load-bearing invariants are written up in ADR 0001, with regression coverage for the mismatched-amounts reproduction and the no-op-pass guard.

GiftCardOrCouponForm — submit no-op and error-clear loop

Two regressions from the rapid-form v4 migration (#779):

1. Submit was a silent no-op for a non-required field. rapid-form v4's form-level auto-discovery only tracks fields that are required or carry a validation config, so a required={false} input — which is what mfe-checkout renders — left values[type] undefined and handleSubmit returned early, issuing no API request. handleSubmit now reads the typed code straight from the form DOM via form.elements.namedItem, which is authoritative regardless of rapid-form wiring or timing. The form ref also passes resetOnSubmit: false, so rapid-form no longer attaches a native submit listener that wiped the input before React's onSubmit ran — the field is preserved on failure and cleared only by our own form.reset() on success.

2. The error-clearing effect looped. It re-dispatched a freshly filtered array every run; because errors is a dependency, the new reference re-fired the effect indefinitely (React 19 "Maximum update depth exceeded"). It now bails when filtering changes nothing, so it converges.

Adds GiftCardOrCouponForm.integration.spec.tsx, which exercises the real rapid-form wiring — the pre-existing spec mocks rapid-form, which is what hid both bugs.

Notes

Targets v5.0.0 rather than main, which is the v4 line.

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