Fix dangling requests when server unexpectedly shuts down

If the client makes a request to the server and while waiting for the response the server shuts down, the client hangs. Make sure that when the client's message loops ends due to the server's output it's reading from ends, any pending requests are notified.

Author: stephentoub

src/ModelContextProtocol/Shared/McpSession.cs

@@ -174,6 +174,14 @@ await _transport.SendMessageAsync(new JsonRpcError
             // Normal shutdown
             _logger.EndpointMessageProcessingCancelled(EndpointName);
         }
+        finally
+        {
+            // Fail any pending requests, as they'll never be satisfied.
+            foreach (var entry in _pendingRequests)
+            {
+                entry.Value.TrySetException(new InvalidOperationException("The server shut down unexpectedly."));
+            }
+        }
     }
 
     private async Task HandleMessageAsync(IJsonRpcMessage message, CancellationToken cancellationToken)