test: add E2E JetStream push consumers, ACK policies, retention modes, ordered, mirror, source

Adds 8 new E2E tests to JetStreamTests.cs (tests 11-18) covering push
consumer config, AckNone/AckAll policies, Interest/WorkQueue retention,
ordered consumers, mirror streams, and source streams.

Fixes three server gaps exposed by the new tests: mirror JSON parsing
(deliver_subject and mirror object fields were silently ignored in stream
and consumer API handlers), and deliver_subject omitted from consumer
info wire format. Also fixes ShutdownDrainTests to use TaskCompletionSource
on ConnectionDisconnected instead of a Task.Delay poll loop.

src/NATS.Server/JetStream/Api/Handlers/ConsumerApiHandlers.cs

@@ -308,6 +308,18 @@ public static class ConsumerApiHandlers
             if (configEl.TryGetProperty("ephemeral", out var ephemeralEl) && ephemeralEl.ValueKind == JsonValueKind.True)
                 config.Ephemeral = true;
 
+            // Go: consumer.go — deliver_subject marks a consumer as push-based.
+            // Reference: server/consumer.go:deliverSubject field on ConsumerConfig
+            if (configEl.TryGetProperty("deliver_subject", out var deliverSubjectEl))
+            {
+                var ds = deliverSubjectEl.GetString();
+                if (!string.IsNullOrWhiteSpace(ds))
+                {
+                    config.DeliverSubject = ds;
+                    config.Push = true; // presence of deliver_subject implies push mode
+                }
+            }
+
             if (configEl.TryGetProperty("push", out var pushEl) && pushEl.ValueKind == JsonValueKind.True)
                 config.Push = true;
 

src/NATS.Server/JetStream/Api/Handlers/StreamApiHandlers.cs

@@ -516,6 +516,16 @@ public static class StreamApiHandlers
                     config.Storage = StorageType.Memory;
             }
 
+            // Go: stream.go — mirror field is a StreamSource object with at minimum a "name" key.
+            // Reference: server/stream.go:NormalizeConfig mirror handling
+            if (root.TryGetProperty("mirror", out var mirrorEl))
+            {
+                if (mirrorEl.ValueKind == JsonValueKind.Object && mirrorEl.TryGetProperty("name", out var mirrorNameEl))
+                    config.Mirror = mirrorNameEl.GetString();
+                else if (mirrorEl.ValueKind == JsonValueKind.String)
+                    config.Mirror = mirrorEl.GetString();
+            }
+
             if (root.TryGetProperty("source", out var sourceEl))
                 config.Source = sourceEl.GetString();
 

src/NATS.Server/JetStream/Api/JetStreamApiResponse.cs

@@ -157,6 +157,8 @@ public sealed class JetStreamApiResponse
         max_deliver = c.MaxDeliver,
         max_ack_pending = c.MaxAckPending,
         filter_subject = c.FilterSubject,
+        // Go: consumer.go — deliver_subject present for push consumers
+        deliver_subject = string.IsNullOrEmpty(c.DeliverSubject) ? null : c.DeliverSubject,
     };
 
     public static JetStreamApiResponse NotFound(string subject) => new()