Stop migrating Cursor sandbox settings (#39325)

## What changed

- Ignore `.cursor/sandbox.json` during external agent config migration.
- Continue importing supported settings from `.cursor/cli-config.json`.

## Testing

- Update app-server migration coverage to verify that environment variables from `cli-config.json` are imported.

GitOrigin-RevId: 6094f1cb6f698392d58056f826da21c1f1389a1b

codex-rs/external-agent-migration/src/migration_source.rs

@@ -98,12 +98,11 @@ impl ExternalAgentSource {
 
     pub(super) fn effective_settings(
         self,
-        source_config_dir: &Path,
         source_settings: &Path,
     ) -> io::Result<Option<JsonValue>> {
         match self {
             Self::Cla => ClaSource::effective_settings(source_settings),
-            Self::Cur => CurSource::effective_settings(source_config_dir, source_settings),
+            Self::Cur => CurSource::effective_settings(source_settings),
         }
     }