Fix plugin list workspace settings test isolation (#20086)

Fixes test that often fails locally when running `cargo test`
- Add an app-server test helper that combines managed-config isolation
with custom env overrides.
- Isolate `HOME` / `USERPROFILE` in plugin-list workspace settings tests
so host home marketplaces do not affect results.

codex-rs/app-server/tests/common/mcp_process.rs

@@ -118,6 +118,15 @@ impl McpProcess {
         Self::new_with_env(codex_home, &[(DISABLE_MANAGED_CONFIG_ENV_VAR, Some("1"))]).await
     }
 
+    pub async fn new_without_managed_config_with_env(
+        codex_home: &Path,
+        env_overrides: &[(&str, Option<&str>)],
+    ) -> anyhow::Result<Self> {
+        let mut all_env_overrides = vec![(DISABLE_MANAGED_CONFIG_ENV_VAR, Some("1"))];
+        all_env_overrides.extend_from_slice(env_overrides);
+        Self::new_with_env(codex_home, &all_env_overrides).await
+    }
+
     pub async fn new_with_plugin_startup_tasks(codex_home: &Path) -> anyhow::Result<Self> {
         Self::new_with_env_and_args(codex_home, &[], &[]).await
     }