[feature]: MPP compatibility for single hop blinded paths

[ziggie1984]

Right now when we send to a blinded route with only 1 hop after the intropoint we immediatly fail the payment and won't retry. The main reason for this was that we did not want to penalize the introduction node pair to the final hop because it could be gamed potentially.

However I think in combination of making the MC data for blinded paths volatile (not persisting the data) we can also get rid of this check as well:

lnd/routing/result_interpretation.go

Lines 514 to 518 in e30e43e

	// Note that if LND is extended to support multiple blinded
	// routes, this will terminate the payment without re-trying
	// the other routes.
	if introIdx == len(route.hops.Val)-1 {
	i.finalFailureReason = &reasonError

[ziggie1984]

Should be done after #9321