CB-527/CB-528: bound AMQP prefetch and confirm publishes before claiming durable

CB-527: basicQos(prefetch) on the consume channel before basicConsume, configurable
via broker.prefetch (default 32), so an undrained inbox backlog stays on the broker
instead of growing the JVM heap without limit.

CB-528: publish moves to its own confirm-mode channel with mandatory=true and a
return listener, so an unroutable or unconfirmed reply now throws instead of
vanishing silently. The confirm callback checks the per-message returned flag
(set by the return listener, which the broker always fires before the matching
confirm) so an acked-but-returned publish is still reported as a failure. The ack
path stays on its own channel/lock and never waits on a publish confirm.

bridged/src/test/java/dev/ltms/bridged/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.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 /**
@@ -166,6 +167,88 @@ class AmqpReplyInboxContractTest {
         }
     }
 
+    @Test
+    void prefetchBoundsTheHeldBacklog() throws Exception {
+        String target = "worker-prefetch-" + System.nanoTime();
+        int prefetch = 4;
+        int published = 10;
+        try (AmqpReplyInbox inbox = new AmqpReplyInbox(newConnection(), prefetch);
+             Connection inspect = newConnection()) {
+            inbox.own(target);
+            for (int i = 0; i < published; i++) {
+                inbox.publish(target, "m" + i, "payload " + i);
+            }
+            awaitHeldAtLeast(inbox, target, prefetch);
+
+            long depth;
+            try (Channel ch = inspect.createChannel()) {
+                depth = ch.queueDeclarePassive(queueName(target)).getMessageCount();
+            }
+            assertTrue(depth >= published - prefetch,
+                    "broker should still hold at least " + (published - prefetch)
+                            + " undelivered messages behind a prefetch of " + prefetch + ", saw " + depth);
+
+            drainUntilEmpty(inbox, target, inspect);
+        }
+    }
+
+    @Test
+    void unroutablePublishReportsFailureNotSilentSuccess() throws Exception {
+        String target = "worker-unroutable-" + System.nanoTime();
+        try (AmqpReplyInbox inbox = AmqpReplyInbox.open(uri())) {
+            // Deliberately never own(target): the queue is never declared, so the default-exchange
+            // route to agent.<target>.inbox does not exist and the broker must return the publish.
+            IllegalStateException ex = assertThrows(IllegalStateException.class,
+                    () -> inbox.publish(target, "m1", "nobody home"));
+            assertTrue(ex.getMessage() != null && ex.getMessage().toLowerCase().contains("unroutable"),
+                    "expected an unroutable-publish failure, got: " + ex.getMessage());
+        }
+    }
+
+    @Test
+    void confirmedPublishDeliversNormally() throws Exception {
+        String target = "worker-confirm-" + System.nanoTime();
+        try (AmqpReplyInbox inbox = AmqpReplyInbox.open(uri())) {
+            inbox.own(target);
+            inbox.publish(target, "m1", "confirmed delivery"); // must return normally: routed and confirmed
+
+            List<ReplyInbox.InboxMessage> got = awaitPeek(inbox, target);
+            assertEquals(1, got.size());
+            assertEquals("confirmed delivery", got.getFirst().content());
+            inbox.ack(target, "m1");
+        }
+    }
+
+    /** Poll peek until at least {@code n} replies for {@code target} are held, or ~10s elapse. */
+    @SuppressWarnings("BusyWait")
+    private static void awaitHeldAtLeast(AmqpReplyInbox inbox, String target, int n) throws InterruptedException {
+        long deadline = System.nanoTime() + TimeUnit.SECONDS.toNanos(10);
+        while (inbox.peek(target).size() < n && System.nanoTime() < deadline) {
+            Thread.sleep(50);
+        }
+    }
+
+    /**
+     * Repeatedly ack whatever is currently held (freeing prefetch slots for the next delivery) until
+     * both the local snapshot and the broker's own queue depth are empty, or ~10s elapse.
+     */
+    @SuppressWarnings("BusyWait")
+    private static void drainUntilEmpty(AmqpReplyInbox inbox, String target, Connection inspect) throws Exception {
+        long deadline = System.nanoTime() + TimeUnit.SECONDS.toNanos(10);
+        while (System.nanoTime() < deadline) {
+            for (ReplyInbox.InboxMessage msg : inbox.peek(target)) {
+                inbox.ack(target, msg.msgId());
+            }
+            try (Channel ch = inspect.createChannel()) {
+                if (ch.queueDeclarePassive(queueName(target)).getMessageCount() == 0 && inbox.peek(target).isEmpty()) {
+                    return;
+                }
+            }
+            Thread.sleep(50);
+        }
+        throw new AssertionError("did not drain " + target + " to empty within the deadline");
+    }
+
     /** Poll peek (broker delivery is async) until a reply for {@code target} appears or ~10s elapse. */
     @SuppressWarnings("BusyWait") // deliberate poll for async broker delivery, bounded by the deadline
     private static List<ReplyInbox.InboxMessage> awaitPeek(AmqpReplyInbox inbox, String target)