test(go): assert ping echo in JSON output; comment ping fallback

TestRunPingJSON now verifies the fake gateway's echoed text appears in
the serialised reply body, catching any future wiring regression that
maps PingRaw to the wrong proto field.  runPing gains a one-line comment
explaining why DiagnosticMessage carries the echo, why the kind-string
fallback exists, and why writeCommandOutput is not reused on the
plain-text path.

clients/go/cmd/mxgw-go/main_test.go

@@ -255,6 +255,13 @@ func TestRunPingJSON(t *testing.T) {
 	if out.Command != "ping" {
 		t.Fatalf("command = %q, want %q", out.Command, "ping")
 	}
+	// The fake gateway echoes "pong:<message>" in diagnostic_message; verify the
+	// echo appears in the serialised reply so a future regression that wired
+	// PingRaw to the wrong proto field would be caught here.
+	replyStr := string(out.Reply)
+	if !strings.Contains(replyStr, "pong:hello") {
+		t.Fatalf("ping JSON reply missing echoed message %q; reply = %s", "pong:hello", replyStr)
+	}
 }
 
 // TestRunPingRequiresSessionID verifies the ping subcommand rejects missing session-id.