feat: complete remaining jetstream parity implementation plan

tests/NATS.Server.Tests/JetStreamPolicyValidationTests.cs

@@ -0,0 +1,22 @@
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream.Validation;
+
+namespace NATS.Server.Tests;
+
+public class JetStreamPolicyValidationTests
+{
+    [Fact]
+    public void Validator_rejects_invalid_policy_combinations()
+    {
+        var cfg = new StreamConfig
+        {
+            Name = "S",
+            Subjects = ["s.*"],
+            Retention = RetentionPolicy.WorkQueue,
+            MaxConsumers = 0,
+        };
+
+        var result = JetStreamConfigValidator.Validate(cfg);
+        result.IsValid.ShouldBeFalse();
+    }
+}