test(app-server): stabilize app/list thread feature-flag test by using file-backed MCP OAuth creds (#11521)

## Why

`suite::v2::app_list::list_apps_uses_thread_feature_flag_when_thread_id_is_provided`
has been flaky in CI. The test exercises `thread/start`, which
initializes `codex_apps`. In CI/Linux, that path can reach OS
keyring-backed MCP OAuth credential lookup (`Codex MCP Credentials`) and
intermittently abort the MCP process (observed stack overflow in
`zbus`), causing the test to fail before the assertion logic runs.

## What Changed

- Updated the test config in
`codex-rs/app-server/tests/suite/v2/app_list.rs` to set
`mcp_oauth_credentials_store = "file"` in both relevant config-writing
paths:
- The in-test config override inside
`list_apps_uses_thread_feature_flag_when_thread_id_is_provided`
- `write_connectors_config(...)`, which is used by the v2 `app_list`
test suite
- This keeps test coverage focused on thread-scoped app feature flags
while removing OS keyring/DBus dependency from this test path.

## How It Was Verified

- `cargo test -p codex-app-server`
- `cargo test -p codex-app-server
list_apps_uses_thread_feature_flag_when_thread_id_is_provided --
--nocapture`

codex-rs/app-server/tests/suite/v2/app_list.rs

@@ -120,6 +120,7 @@ async fn list_apps_uses_thread_feature_flag_when_thread_id_is_provided() -> Resu
         format!(
             r#"
 chatgpt_base_url = "{server_url}"
+mcp_oauth_credentials_store = "file"
 
 [features]
 connectors = false
@@ -791,6 +792,7 @@ fn write_connectors_config(codex_home: &std::path::Path, base_url: &str) -> std:
         format!(
             r#"
 chatgpt_base_url = "{base_url}"
+mcp_oauth_credentials_store = "file"
 
 [features]
 connectors = true