Refresh remote plugin cache on auth changes (#20265)

## Summary
- Refresh the remote installed-plugin cache after login/logout instead
of keying it by account or eagerly clearing it.
- Reuse the existing single-flight remote installed refresh loop so
newer queued auth refreshes replace older pending requests and the API
result eventually overwrites or clears the cache.
- Keep derived plugin/skills cache and MCP refresh side effects behind
the existing effective-plugin-changed task when the refreshed installed
state changes.
- Leave `clear_plugin_related_caches` scoped to derived plugin/skills
caches so share mutations do not drop remote installed plugins.

## Tests
- `cargo fmt --all --manifest-path codex-rs/Cargo.toml` (passes; stable
rustfmt warns that `imports_granularity = Item` is nightly-only)
- `cargo test -p codex-core-plugins remote_installed_cache`
- `cargo test -p codex-app-server
skills_list_loads_remote_installed_plugin_skills_from_cache`

codex-rs/core-plugins/src/manager.rs

@@ -387,8 +387,6 @@ pub struct PluginsManager {
     configured_marketplace_upgrade_state: RwLock<ConfiguredMarketplaceUpgradeState>,
     non_curated_cache_refresh_state: RwLock<NonCuratedCacheRefreshState>,
     cached_enabled_outcome: RwLock<Option<CachedPluginLoadOutcome>>,
-    // TODO(remote plugins): reset this cache when ChatGPT auth/account state changes so stale
-    // remote installed state cannot remain effective for a different account.
     remote_installed_plugins_cache: RwLock<Option<Vec<RemoteInstalledPlugin>>>,
     remote_installed_plugins_cache_refresh_state: RwLock<RemoteInstalledPluginsCacheRefreshState>,
     remote_sync_lock: Semaphore,