[nat64] schedule evaluation of policy when Nat64PrefixManager::Discover() fails (#11654)

Previously we were relying on the platform to call
`otPlatInfraIfDiscoverNat64PrefixDone` to trigger the evaluation of
NAT64 prefix. Since
https://github.com/openthread/openthread/pull/11481, we're no longer
doing proactive DNS-based prefix discoveries and it never calls the
callback `otPlatInfraIfDiscoverNat64PrefixDone`.

That means NAT64 evaluation can only happen when
`RoutingManager::EvaluateRoutingPolicy` is triggered, which can cause
a at most 3-minute delay (`kRaBeaconInterval`), between enabling NAT64
feature and the feature starts to work.

This commit triggers the evaluation of routing policy when
`Nat64PrefixManager::Discover()` fails to ensure the feature starts to
work on time.

src/core/border_router/routing_manager.cpp

@@ -4145,6 +4145,7 @@ void RoutingManager::Nat64PrefixManager::Discover(void)
     else
     {
         LogWarn("Failed to discover infraif NAT64 prefix: %s", ErrorToString(error));
+        Get<RoutingManager>().ScheduleRoutingPolicyEvaluation(kAfterRandomDelay);
     }
 }