fix: #3615265 The payment landing page fatals for a payer coming back mid-checkout, because it asks kessai for an accessor kessai removed

PaymentInteraction::show() asked the pinned payment for getHostedCheckoutId(), which kessai replaced with getCheckoutSession(). The landing page therefore fataled for any payer returning while a payment was pending. One line, same meaning: a session exists exactly when the payer was sent to a hosted checkout.

The real defect is that the branch had no test, so the provider's rename could not turn anything red here. PaymentInFlightTest covers both sides:

• a pending payment holding a session reports the payment is in flight;
• a pending payment holding none still offers to pay, so the fix cannot degenerate into always reporting in flight.

Both were seen to fail against the old call and pass with the fix.