fix: prevent session hang when system.client_routes table is absent

Pre-load client routes during init() before topology discovery. When the
routes cache is empty (table doesn't exist on older Scylla versions),
buildNodeEndPoint falls back to DefaultEndPoint instead of creating
ClientRoutesEndPoint that would fail to resolve, causing infinite
connection retry loops and session builder hangs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dkropachev

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

@@ -89,7 +89,10 @@ public CompletionStage<Void> init() {
         context
             .getEventBus()
             .register(ControlConnectionReconnectEvent.class, this::onReconnectEvent);
-    return super.init();
+    // Pre-load client routes before topology discovery so that buildNodeEndPoint can decide
+    // whether to use ClientRoutesEndPoint (routes available) or fall back to DefaultEndPoint
+    // (table absent / empty — e.g. older Scylla versions without system.client_routes).
+    return queryAndResolveRoutes(false).thenCompose(ignored -> super.init());
   }
 
   /** Returns the {@link ClientRoutesConfig} this handler was built from. */
@@ -242,6 +245,12 @@ protected EndPoint buildNodeEndPoint(
       @NonNull AdminRow row,
       @Nullable InetSocketAddress broadcastRpcAddress,
       @NonNull EndPoint localEndPoint) {
+    // If the routes cache is empty (e.g. system.client_routes does not exist on this server
+    // version), fall back to the default endpoint resolution so that the session can still
+    // function normally.
+    if (resolvedRoutesCache.get().isEmpty()) {
+      return super.buildNodeEndPoint(row, broadcastRpcAddress, localEndPoint);
+    }
     UUID hostId = Objects.requireNonNull(row.getUuid("host_id"));
     InetAddress broadcastInetAddress = null;
     if (broadcastRpcAddress != null) {