fleetd #719: make the foreign-id test reach a colliding sequence number

other.sendAsync had never been called, so other's tasks map was empty and
poll(ticket) returned null regardless of whether the nonce existed — the
test passed against an empty map, not against a colliding id. Mint once on
other so it reaches the same sequence number as the first instance, making
the test exercise the actual collision the nonce guards against.

fleetd/src/test/java/dev/ltms/fleet/msg/MessageServiceTest.java

@@ -911,6 +911,9 @@ class MessageServiceTest {
     void foreignInstanceTicketDoesNotResolve() {
         MessageService other = newIndependentInstance();
         String ticket = messages.sendAsync(T, "task on first instance", null, null);
+        // `other` must reach the same sequence number, or this test passes against an empty map
+        // instead of against a colliding id.
+        other.sendAsync(T, "task on second instance", null, null);
 
         // control: the id resolves in the instance that minted it, so a null below cannot be
         // explained by broken plumbing — only by the ticket being foreign to `other`.