test: isolate box factory policy env in tests

src/box_factory/mod.rs

@@ -518,6 +518,7 @@ mod tests {
 
     #[test]
     fn test_default_policy_is_strict_plugin_first() {
+        let prev = std::env::var("NYASH_BOX_FACTORY_POLICY").ok();
         // Ensure NYASH_BOX_FACTORY_POLICY is not set
         std::env::remove_var("NYASH_BOX_FACTORY_POLICY");
 
@@ -527,10 +528,16 @@ mod tests {
             FactoryPolicy::StrictPluginFirst,
             "Default policy should be StrictPluginFirst"
         );
+
+        if let Some(v) = prev {
+            std::env::set_var("NYASH_BOX_FACTORY_POLICY", v);
+        }
     }
 
     #[test]
     fn test_env_policy_override() {
+        let prev = std::env::var("NYASH_BOX_FACTORY_POLICY").ok();
+
         // Test builtin_first override
         std::env::set_var("NYASH_BOX_FACTORY_POLICY", "builtin_first");
         let registry = UnifiedBoxRegistry::with_env_policy();
@@ -547,7 +554,11 @@ mod tests {
         assert_eq!(registry.get_policy(), FactoryPolicy::StrictPluginFirst);
 
         // Cleanup
-        std::env::remove_var("NYASH_BOX_FACTORY_POLICY");
+        if let Some(v) = prev {
+            std::env::set_var("NYASH_BOX_FACTORY_POLICY", v);
+        } else {
+            std::env::remove_var("NYASH_BOX_FACTORY_POLICY");
+        }
     }
 
     #[test]