feat: add jetstream mirror and source orchestration

tests/NATS.Server.Tests/JetStreamApiFixture.cs

@@ -61,6 +61,18 @@ internal sealed class JetStreamApiFixture : IAsyncDisposable
         return fixture;
     }
 
+    public static async Task<JetStreamApiFixture> StartWithMirrorSetupAsync()
+    {
+        var fixture = await StartWithStreamAsync("ORDERS", "orders.*");
+        _ = fixture._streamManager.CreateOrUpdate(new StreamConfig
+        {
+            Name = "ORDERS_MIRROR",
+            Subjects = ["orders.mirror.*"],
+            Mirror = "ORDERS",
+        });
+        return fixture;
+    }
+
     public Task<PubAck> PublishAndGetAckAsync(string subject, string payload, string? msgId = null, bool expectError = false)
     {
         if (_publisher.TryCapture(subject, Encoding.UTF8.GetBytes(payload), msgId, out var ack))
@@ -81,6 +93,11 @@ internal sealed class JetStreamApiFixture : IAsyncDisposable
         throw new InvalidOperationException($"No stream matched subject '{subject}'.");
     }
 
+    public Task<PubAck> PublishAndGetAckAsync(string streamName, string subject, string payload)
+    {
+        return PublishAndGetAckAsync(subject, payload);
+    }
+
     public Task<JetStreamApiResponse> RequestLocalAsync(string subject, string payload)
     {
         return Task.FromResult(_router.Route(subject, Encoding.UTF8.GetBytes(payload)));
@@ -122,5 +139,17 @@ internal sealed class JetStreamApiFixture : IAsyncDisposable
         return Task.FromResult(frame);
     }
 
+    public async Task WaitForMirrorSyncAsync(string streamName)
+    {
+        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(2));
+        while (!timeout.IsCancellationRequested)
+        {
+            var state = await GetStreamStateAsync(streamName);
+            if (state.Messages > 0)
+                return;
+            await Task.Delay(25, timeout.Token).ContinueWith(_ => { }, TaskScheduler.Default);
+        }
+    }
+
     public ValueTask DisposeAsync() => ValueTask.CompletedTask;
 }