feat: phase C jetstream depth test parity — 34 new tests across 7 subsystems

Stream lifecycle, publish/ack, consumer delivery, retention policy,
API endpoints, cluster formation, and leader failover tests ported
from Go nats-server reference. 1006 total tests passing.

src/NATS.Server/JetStream/Validation/JetStreamConfigValidator.cs

@@ -1,3 +1,4 @@
+using NATS.Server.Configuration;
 using NATS.Server.JetStream.Models;
 
 namespace NATS.Server.JetStream.Validation;
@@ -20,6 +21,27 @@ public static class JetStreamConfigValidator
 
         return ValidationResult.Valid();
     }
+
+    /// <summary>
+    /// Validates JetStream cluster configuration requirements.
+    /// When JetStream is enabled and clustering is configured (Cluster.Port > 0),
+    /// both server_name and cluster.name must be set.
+    /// Reference: Go server/jetstream.go validateOptions (line ~2822-2831).
+    /// </summary>
+    public static ValidationResult ValidateClusterConfig(NatsOptions options)
+    {
+        // If JetStream is not enabled or not clustered, no cluster-specific checks needed.
+        if (options.JetStream == null || options.Cluster == null || options.Cluster.Port == 0)
+            return ValidationResult.Valid();
+
+        if (string.IsNullOrEmpty(options.ServerName))
+            return ValidationResult.Invalid("jetstream cluster requires `server_name` to be set");
+
+        if (string.IsNullOrEmpty(options.Cluster.Name))
+            return ValidationResult.Invalid("jetstream cluster requires `cluster.name` to be set");
+
+        return ValidationResult.Valid();
+    }
 }
 
 public sealed class ValidationResult