feat: add jetstream config validation models

tests/NATS.Server.Tests/JetStreamConfigValidationTests.cs

@@ -0,0 +1,15 @@
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream.Validation;
+
+namespace NATS.Server.Tests;
+
+public class JetStreamConfigValidationTests
+{
+    [Fact]
+    public void Stream_requires_name_and_subjects()
+    {
+        var config = new StreamConfig { Name = "", Subjects = [] };
+        var result = JetStreamConfigValidator.Validate(config);
+        result.IsValid.ShouldBeFalse();
+    }
+}