fix: unregister event bus listener on init() failure

If super.init() or the initial route query fails, unregister the
ClientRoutesUpdateEvent listener to prevent a leak when closeAsync()
is not called by the caller on failure.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dkropachev

core/src/main/java/com/datastax/oss/driver/internal/core/metadata/ClientRoutesTopologyMonitor.java

@@ -165,7 +165,16 @@ public CompletionStage<Void> init() {
     // buildNodeEndPoint (called during the subsequent refreshNodes) can use ClientRoutesEndPoint.
     // If the server does not support CLIENT_ROUTES_CHANGE, the control connection will fail with
     // a ConnectionInitException and the error propagates naturally.
-    return super.init().thenCompose(ignored -> queryClientRoutesAndCache(null, null));
+    return super.init()
+        .thenCompose(ignored -> queryClientRoutesAndCache(null, null))
+        .whenComplete(
+            (result, error) -> {
+              if (error != null) {
+                context
+                    .getEventBus()
+                    .unregister(clientRoutesUpdateKey, ClientRoutesUpdateEvent.class);
+              }
+            });
   }
 
   /** Returns the {@link ClientRoutesConfig} this handler was built from. */