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.JetStream.Tests/JetStream/Streams/ConfigUpdateValidationTests.cs

@@ -278,10 +278,10 @@ public class ConfigUpdateValidationTests
         errors.ShouldBeEmpty();
     }
 
-    // Go ref: server/stream.go — RAFT consensus requires an odd number of replicas.
-    // Setting replicas to an even number must be rejected.
+    // Go ref: server/stream.go — Go server supports even replica counts (e.g., R2).
+    // Even replicas should be accepted by config update validation.
     [Fact]
-    public void ValidateConfigUpdate_rejects_even_replicas()
+    public void ValidateConfigUpdate_accepts_even_replicas()
     {
         var existing = new StreamConfig
         {
@@ -300,7 +300,7 @@ public class ConfigUpdateValidationTests
 
         var errors = StreamManager.ValidateConfigUpdate(existing, proposed);
 
-        errors.ShouldContain(e => e.Contains("replicas must be odd"));
+        errors.ShouldBeEmpty();
     }
 
     // Go ref: server/stream.go:1500-1600 (stream.update) — integration via StreamManager.

tests/NATS.Server.JetStream.Tests/JetStreamApiProtocolIntegrationTests.cs

@@ -12,7 +12,7 @@ public class JetStreamApiProtocolIntegrationTests
         await using var server = await ServerFixture.StartJetStreamEnabledAsync();
         var response = await server.RequestAsync("$JS.API.INFO", "{}", timeoutMs: 1000);
 
-        response.ShouldContain("\"error\"");
+        response.ShouldContain("\"streams\"");
     }
 }
 

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.