fleetd #437: pin the ack hit/miss contract in the AMQP contract test

AmqpReplyInboxContractTest is the one contract-group class CI actually
runs, and it never asserted on ack()'s return value at all — so the
exact defect this ticket fixes (reporting success for an ack that
removed nothing) was unpinned in the adapter fleetd runs live.

Add ackReportsHitVsMissAgainstARealBroker: a msgId never held for an
owned target returns false without throwing, a real held reply returns
true and is removed, and acking the same msgId again returns false.
Ran against both broker modes the class supports: Testcontainers
(AMQP_URI unset) and an external broker via AMQP_URI (the CI shape,
using a disposable container — not the shared local LavinMQ instance).

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

@@ -16,6 +16,7 @@ import java.util.List;
 import java.util.concurrent.TimeUnit;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
@@ -221,6 +222,31 @@ class AmqpReplyInboxContractTest {
         }
     }
 
+    @Test
+    void ackReportsHitVsMissAgainstARealBroker() throws Exception {
+        // fleetd #437: fleet_ack said "acknowledged <msgId>" for a message it never touched,
+        // because ReplyInbox.ack() (void) could not tell a hit from a miss. Pin the fixed
+        // boolean contract against a real broker — the adapter fleetd actually runs live.
+        String target = "worker-ack-contract-" + System.nanoTime();
+        try (AmqpReplyInbox inbox = AmqpReplyInbox.open(uri())) {
+            inbox.own(target);
+
+            // Never held for this target at all: must report false, not throw.
+            assertFalse(inbox.ack(target, "never-held"),
+                    "acking a msgId never held for an owned target must report false");
+
+            // A real message: first ack removes it and reports true...
+            inbox.publish(target, "m1", "ack me");
+            assertEquals(1, awaitPeek(inbox, target).size(), "the published reply should be held");
+            assertTrue(inbox.ack(target, "m1"), "acking a held reply must report true");
+            assertTrue(inbox.peek(target).isEmpty(), "an acked reply is dropped");
+
+            // ...and the second ack of the SAME msgId has nothing left to remove: false.
+            assertFalse(inbox.ack(target, "m1"),
+                    "acking the same msgId twice must report false the second time");
+        }
+    }
+
     @Test
     void confirmedPublishDeliversNormally() throws Exception {
         String target = "worker-confirm-" + System.nanoTime();