fix(link): recover stale websocket sessions [CI]

backend/fleet/src/Fleet.Link/Transports.cs

@@ -7,6 +7,7 @@ namespace Fleet.Link;
 /// <summary>WebSocket 传输（节点 → 服务端）：文本帧是 JSON-RPC，二进制帧是发件箱。</summary>
 public sealed class WsTransport : IRpcTransport
 {
+    private static readonly TimeSpan s_sendTimeout = TimeSpan.FromSeconds(20);
     private readonly WebSocket _socket;
     private readonly int _maxFrameBytes;
     private readonly SemaphoreSlim _sendLock = new(1, 1);
@@ -19,14 +20,32 @@ public sealed class WsTransport : IRpcTransport
 
     public async ValueTask SendAsync(RpcFrame frame, CancellationToken ct)
     {
-        await _sendLock.WaitAsync(ct);
+        using var timeout = new CancellationTokenSource(s_sendTimeout);
+        using CancellationTokenSource linked = CancellationTokenSource.CreateLinkedTokenSource(ct, timeout.Token);
+        bool lockTaken = false;
         try
         {
-            await _socket.SendAsync(frame.Data, frame.Kind == FrameKind.Text ? WebSocketMessageType.Text : WebSocketMessageType.Binary, endOfMessage: true, ct);
+            await _sendLock.WaitAsync(linked.Token);
+            lockTaken = true;
+            if (_socket.State != WebSocketState.Open)
+            {
+                throw new WebSocketException($"WebSocket is not open (state: {_socket.State})");
+            }
+
+            await _socket.SendAsync(frame.Data, frame.Kind == FrameKind.Text ? WebSocketMessageType.Text : WebSocketMessageType.Binary, endOfMessage: true, linked.Token);
+        }
+        catch (OperationCanceledException) when (timeout.IsCancellationRequested && !ct.IsCancellationRequested)
+        {
+            // A wedged send otherwise holds the lock and prevents heartbeat/reconnect from making progress.
+            _socket.Abort();
+            throw new TimeoutException($"WebSocket send timed out after {s_sendTimeout.TotalSeconds:0} seconds");
         }
         finally
         {
-            _sendLock.Release();
+            if (lockTaken)
+            {
+                _sendLock.Release();
+            }
         }
     }