Let the payment step route on a resolver outcome (pay, no_payment, empty)

The payment interaction always offers to pay, even when the resolver reports there is nothing to pay for, or nothing to act on at all. This adds a route outcome to the payable resolver contract so the step can carry the run on, or show a message, instead of always offering payment.

Changes:

• New PayableResolverInterface::routeOutcome() returns one of pay, no_payment or empty, with a default in PayableResolverBase derived from resolve(): pay when it resolves a Payable, empty otherwise.
• The payment interaction routes on it. no_payment auto-signals a no_payment outcome with no landing page, so a flow carries the run onward. empty shows a configurable message plus the author-configured outcome buttons, with no amount and no Go to payment button. pay is unchanged.
• A new translatable empty_message setting, plus its config schema key.

Covered by a new kernel test, PaymentRouteOutcomeTest, with a small test-only resolver.