core: fix stale curated plugin cache refresh races

[bolinfest]

Why

The plugin/list force-sync path can race app-server startup's curated plugin cache refresh.

Startup was capturing the configured curated plugin IDs from the initial config snapshot. If plugin/list with forceRemoteSync removed curated plugin entries from config.toml while that background refresh was still in flight, the startup task could recreate cache directories for plugins that had just been uninstalled.

That leaves the plugin/list response logically correct but the on-disk cache stale, which matches the flaky Ubuntu arm failure seen in codex-app-server::all suite::v2::plugin_list::plugin_list_force_remote_sync_reconciles_curated_plugin_state while validating #16047.

What

• change codex-rs/core/src/plugins/manager.rs so startup curated-repo refresh rereads the current user config.toml before deciding which curated plugin cache entries to refresh
• factor the configured-plugin parsing so the same logic can be reused from either the config layer stack or the persisted user config value
• add a regression test that verifies curated plugin IDs are read from the latest user config state before cache refresh runs

Testing

• cargo test -p codex-core configured_curated_plugin_ids_from_codex_home_reads_latest_user_config -- --nocapture
• cargo test -p codex-app-server suite::v2::plugin_list::plugin_list_force_remote_sync_reconciles_curated_plugin_state -- --nocapture
• just argument-comment-lint