feat: Added ping command

proto/vantage/v1/vantage.proto

@@ -182,9 +182,22 @@ message ServerCommand {
     RunStepCmd      run_step      = 6;
     CleanupWorkspaceCmd cleanup_workspace = 7;
     OpenProxyCmd open_proxy = 8;
+    PingCmd ping = 9;
   }
 }
 
+// PingCmd is a liveness beat, carrying nothing and requiring no reply.
+//
+// It exists because gRPC keepalive cannot prove what the agent needs to know.
+// Behind an L7 proxy the agent's HTTP/2 connection terminates at the proxy, so
+// keepalive pings are answered by the proxy whether or not the server behind it
+// is still there. A pod that dies leaves the agent blocked in Recv on a stream
+// that will never produce another message and never error — commands are
+// dispatched into it and silently lost. Only traffic that originates at the
+// server itself distinguishes a live stream from an orphaned one.
+message PingCmd {
+}
+
 // CleanupWorkspaceCmd tells the agent to recursively remove the run's working
 // directory once all steps on that server have finished.
 message CleanupWorkspaceCmd {