fix(boot-test): fill {PUBLIC_*} in the startup command like the panel does

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MZxvxViV6QwAtVWT3saGvs

cmd/gbrgames/boottest_test.go

@@ -132,3 +132,18 @@ func TestSteamCacheStampAge(t *testing.T) {
 		t.Error("old, missing or garbled stamps are not fresh")
 	}
 }
+
+func TestBootTestPayloadFillsPublicPlaceholdersInStartupCommand(t *testing.T) {
+	dir := t.TempDir()
+	game := filepath.Join(dir, "example")
+	os.MkdirAll(game, 0o755)
+	os.WriteFile(filepath.Join(game, "game.yaml"), []byte("id: example\nname: Example\ndefaultDockerImage: x\n"+
+		"defaultStartupCommand: ./server -Port={PUBLIC_PORT_7777} -ip={PUBLIC_IP}\ndefaultPorts:\n  - 7777/udp\n"), 0o644)
+	p, err := bootTestPayload(dir, "example", false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if p["startupCommand"] != "./server -Port=7777 -ip=127.0.0.1" {
+		t.Errorf("startupCommand %v", p["startupCommand"])
+	}
+}