Add awaitility module: testing async @Async/@Scheduled/@KafkaListener code without Thread.sleep

Covers replacing Thread.sleep with await() across a void @Async method, an
@EmbeddedKafka-backed @KafkaListener, and an already-running @Scheduled job;
verifies Awaitility 4.3.0's real defaults (10s timeout, 100ms poll interval)
and its default-uncaught-exception-handler swap directly against the jar;
and documents a real pom.xml trap where Boot 4.1.1 split Kafka's
autoconfiguration (spring-boot-kafka) out of spring-kafka itself, which
silently leaves @KafkaListener beans with no running container.

awaitility/src/main/java/com/ankurm/awaitility/Mailbox.java

@@ -0,0 +1,31 @@
+package com.ankurm.awaitility;
+
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
+import org.springframework.stereotype.Component;
+
+/**
+ * Stands in for "the side effect of an asynchronous operation" — a row written by a
+ * background thread that a test (or another part of the application) has no handle to wait on
+ * directly. {@link AsyncGreetingService#sendConfirmation(String)} writes here; nothing returns a
+ * {@code Future} for it to block on, which is the whole reason the test for it needs Awaitility
+ * rather than a blocking {@code get()}.
+ */
+@Component
+public class Mailbox {
+
+    private final ConcurrentMap<String, String> sent = new ConcurrentHashMap<>();
+
+    void record(String orderId, String message) {
+        sent.put(orderId, message);
+    }
+
+    /** Returns {@code null} if nothing has been recorded for this order yet. */
+    public String confirmationFor(String orderId) {
+        return sent.get(orderId);
+    }
+
+    public int size() {
+        return sent.size();
+    }
+}