feat(jetstream): add API leader forwarding and stream purge options (C7+C8)

C7: JetStreamApiRouter now checks leadership before mutating operations.
Non-leader nodes return error code 10003 with a leader_hint field.
JetStreamMetaGroup gains IsLeader() and Leader for cluster-aware routing.

C8: StreamApiHandlers.HandlePurge accepts PurgeRequest options (filter,
seq, keep). StreamManager.PurgeEx implements subject-filtered purge,
sequence-based purge, keep-last-N, and filter+keep combinations.

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

@@ -15,6 +15,7 @@ public sealed class JetStreamApiResponse
     public JetStreamSnapshot? Snapshot { get; init; }
     public JetStreamPullBatch? PullBatch { get; init; }
     public bool Success { get; init; }
+    public ulong Purged { get; init; }
 
     public static JetStreamApiResponse NotFound(string subject) => new()
     {
@@ -40,6 +41,31 @@ public sealed class JetStreamApiResponse
             Description = description,
         },
     };
+
+    /// <summary>
+    /// Returns a not-leader error with code 10003 and a leader_hint.
+    /// Go reference: jetstream_api.go:200-300 — non-leader nodes return this error
+    /// for mutating operations so clients can redirect.
+    /// </summary>
+    public static JetStreamApiResponse NotLeader(string leaderHint) => new()
+    {
+        Error = new JetStreamApiError
+        {
+            Code = 10003,
+            Description = "not leader",
+            LeaderHint = leaderHint,
+        },
+    };
+
+    /// <summary>
+    /// Returns a purge success response with the number of messages purged.
+    /// Go reference: jetstream_api.go:1200-1350 — purge response includes purged count.
+    /// </summary>
+    public static JetStreamApiResponse PurgeResponse(ulong purged) => new()
+    {
+        Success = true,
+        Purged = purged,
+    };
 }
 
 public sealed class JetStreamStreamInfo