Hand the payment step off to kessai's handoff route

Merge request for #3611855.

The Go to payment action now only creates+pins the payment and redirects into kessai's kessai.pay handoff route, which owns the provider round-trip and fires kessai.initiated. The bridge no longer calls initiate() or builds the gateway redirect.

Depends on the kessai handoff route (kessai #3611852); CI here goes green once that lands in kessai 1.x-dev.