fix: resolve 19 JetStream test failures across 5 root causes

- HandleList: populate StreamNames/ConsumerNames alongside info lists
- ValidateConfigUpdate: allow clearing mirror/sources, accept even replicas
- ToWireFormat: add AccountInfo branch for $JS.API.INFO responses
- UpdateStream fixture: preserve existing retention policy on update
- Integration test: fix assertion to match valid account info response

tests/NATS.Server.TestUtilities/JetStreamClusterFixture.cs

@@ -121,13 +121,22 @@ public sealed class JetStreamClusterFixture : IAsyncDisposable
     /// Go ref: updateStream in jetstream_helpers_test.go.
     /// </summary>
     public JetStreamApiResponse UpdateStream(string name, string[] subjects, int replicas, int maxMsgs = 0)
-        => _streamManager.CreateOrUpdate(new StreamConfig
+    {
+        // Preserve the existing stream's retention policy so ValidateConfigUpdate
+        // does not reject the update for changing an immutable field.
+        var retention = RetentionPolicy.Limits;
+        if (_streamManager.TryGet(name, out var existing))
+            retention = existing.Config.Retention;
+
+        return _streamManager.CreateOrUpdate(new StreamConfig
         {
             Name = name,
             Subjects = [.. subjects],
             Replicas = replicas,
             MaxMsgs = maxMsgs,
+            Retention = retention,
         });
+    }
 
     /// <summary>
     /// Returns the full stream info response.