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.

tests/NATS.E2E.Tests/ShutdownDrainTests.cs

@@ -0,0 +1,72 @@
+using NATS.Client.Core;
+using NATS.E2E.Tests.Infrastructure;
+
+namespace NATS.E2E.Tests;
+
+public class ShutdownDrainTests
+{
+    [Fact]
+    public async Task ClientDrain_CompletesInFlightMessages()
+    {
+        await using var server = new NatsServerProcess();
+        await server.StartAsync();
+
+        await using var pub = new NatsConnection(new NatsOpts { Url = $"nats://127.0.0.1:{server.Port}" });
+        var sub = new NatsConnection(new NatsOpts { Url = $"nats://127.0.0.1:{server.Port}" });
+        await using var _ = sub;
+
+        await pub.ConnectAsync();
+        await sub.ConnectAsync();
+
+        await using var subscription = await sub.SubscribeCoreAsync<string>("e2e.drain.>");
+        await sub.PingAsync();
+
+        for (var i = 0; i < 10; i++)
+            await pub.PublishAsync($"e2e.drain.{i}", $"msg{i}");
+        await pub.PingAsync();
+
+        var received = new List<string?>();
+        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));
+
+        for (var i = 0; i < 10; i++)
+        {
+            var msg = await subscription.Msgs.ReadAsync(cts.Token);
+            received.Add(msg.Data);
+        }
+
+        received.Count.ShouldBe(10);
+
+        await sub.DisposeAsync();
+    }
+
+    [Fact]
+    public async Task ServerShutdown_ClientDetectsDisconnection()
+    {
+        var server = new NatsServerProcess();
+        await server.StartAsync();
+
+        await using var client = new NatsConnection(new NatsOpts
+        {
+            Url = $"nats://127.0.0.1:{server.Port}",
+            MaxReconnectRetry = 0,
+        });
+        await client.ConnectAsync();
+        await client.PingAsync();
+
+        client.ConnectionState.ShouldBe(NatsConnectionState.Open);
+
+        var disconnected = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+        client.ConnectionDisconnected += (_, _) =>
+        {
+            disconnected.TrySetResult();
+            return ValueTask.CompletedTask;
+        };
+
+        await server.DisposeAsync();
+
+        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+        await disconnected.Task.WaitAsync(cts.Token);
+
+        client.ConnectionState.ShouldNotBe(NatsConnectionState.Open);
+    }
+}