feat: Added ping command

internal/grpc/pb/vantage.pb.go

@@ -166,8 +166,14 @@ type ServerCommand struct {
 	RunStep          *RunStepCmd          `json:"run_step,omitempty"`
 	CleanupWorkspace *CleanupWorkspaceCmd `json:"cleanup_workspace,omitempty"`
 	OpenProxy        *OpenProxyCmd        `json:"open_proxy,omitempty"`
+	Ping             *PingCmd             `json:"ping,omitempty"`
 }
 
+// PingCmd is a server-originated liveness beat. It carries nothing and expects
+// no reply: its arrival is the entire message. See the .proto for why gRPC
+// keepalive is not sufficient on its own.
+type PingCmd struct{}
+
 
 
 type CleanupWorkspaceCmd struct {