[优化] 重构升级处理逻辑，添加备份二进制文件移除功能，更新相关测试以验证新行为

openflare_agent/internal/updater/restart_unix_test.go

@@ -0,0 +1,15 @@
+//go:build !windows
+
+package updater
+
+import (
+	"path/filepath"
+	"testing"
+)
+
+func TestRemoveBackupBinaryIgnoresMissingFile(t *testing.T) {
+	backupPath := filepath.Join(t.TempDir(), "openflare-agent.bak")
+	if err := removeBackupBinary(backupPath); err != nil {
+		t.Fatalf("expected missing backup cleanup to be ignored: %v", err)
+	}
+}