feat: add source/mirror info reporting for monitoring (Gap 4.10)

Add GetMirrorInfo/GetSourceInfo methods to MirrorCoordinator and
SourceCoordinator returning structured MirrorInfoResponse/SourceInfoResponse
records (Name, Lag, Active ms, FilterSubject, Error). Wire both into
StreamManager.GetMirrorInfo and GetSourceInfos for the monitoring API path.

src/NATS.Server/JetStream/MirrorSource/MirrorCoordinator.cs

@@ -297,6 +297,22 @@ public sealed class MirrorCoordinator : IAsyncDisposable
         };
     }
 
+    /// <summary>
+    /// Returns a structured monitoring response for this mirror.
+    /// Go reference: server/stream.go:2739-2743 (mirrorInfo building StreamSourceInfo)
+    /// </summary>
+    public MirrorInfoResponse GetMirrorInfo(string streamName)
+    {
+        var report = GetHealthReport();
+        return new MirrorInfoResponse
+        {
+            Name = streamName,
+            Lag = report.Lag,
+            Active = report.LastSyncUtc != default ? (long)(DateTime.UtcNow - report.LastSyncUtc).TotalMilliseconds : -1,
+            Error = ErrorMessage,
+        };
+    }
+
     public async ValueTask DisposeAsync()
     {
         await StopAsync();
@@ -462,3 +478,25 @@ public sealed record MirrorHealthReport
     public bool IsRunning { get; init; }
     public bool IsStalled { get; init; }
 }
+
+/// <summary>
+/// Structured monitoring response for a mirror, as returned by the JetStream monitoring API.
+/// Go reference: server/stream.go:2739-2743 (mirrorInfo / StreamSourceInfo)
+/// </summary>
+public sealed record MirrorInfoResponse
+{
+    /// <summary>Name of the stream being mirrored.</summary>
+    public string Name { get; init; } = string.Empty;
+
+    /// <summary>Number of messages behind the origin stream.</summary>
+    public ulong Lag { get; init; }
+
+    /// <summary>
+    /// Milliseconds since the last successful sync.
+    /// -1 when the mirror has never synced.
+    /// </summary>
+    public long Active { get; init; } = -1;
+
+    /// <summary>Current error message, or null if no error.</summary>
+    public string? Error { get; init; }
+}

src/NATS.Server/JetStream/MirrorSource/SourceCoordinator.cs

@@ -295,6 +295,23 @@ public sealed class SourceCoordinator : IAsyncDisposable
         };
     }
 
+    /// <summary>
+    /// Returns a structured monitoring response for this source.
+    /// Go reference: server/stream.go:2687-2736 (sourcesInfo / StreamSourceInfo)
+    /// </summary>
+    public SourceInfoResponse GetSourceInfo()
+    {
+        var report = GetHealthReport();
+        return new SourceInfoResponse
+        {
+            Name = _sourceConfig.Name,
+            Lag = report.Lag,
+            Active = report.LastSyncUtc != default ? (long)(DateTime.UtcNow - report.LastSyncUtc).TotalMilliseconds : -1,
+            FilterSubject = _sourceConfig.FilterSubject,
+            Error = null, // SourceCoordinator doesn't have error state tracking yet
+        };
+    }
+
     public async ValueTask DisposeAsync()
     {
         await StopAsync();
@@ -539,3 +556,28 @@ public sealed record SourceHealthReport
     public long FilteredOutCount { get; init; }
     public long DeduplicatedCount { get; init; }
 }
+
+/// <summary>
+/// Structured monitoring response for a source, as returned by the JetStream monitoring API.
+/// Go reference: server/stream.go:2687-2736 (sourcesInfo / StreamSourceInfo)
+/// </summary>
+public sealed record SourceInfoResponse
+{
+    /// <summary>Name of the origin stream.</summary>
+    public string Name { get; init; } = string.Empty;
+
+    /// <summary>Number of messages behind the origin stream.</summary>
+    public ulong Lag { get; init; }
+
+    /// <summary>
+    /// Milliseconds since the last successful sync.
+    /// -1 when the source has never synced.
+    /// </summary>
+    public long Active { get; init; } = -1;
+
+    /// <summary>Optional subject filter applied by this source coordinator.</summary>
+    public string? FilterSubject { get; init; }
+
+    /// <summary>Current error message, or null if no error.</summary>
+    public string? Error { get; init; }
+}