Function setup_frontend_plugins_post_build 

pub async fn setup_frontend_plugins_post_build(
    _plugins: &mut Plugins,
    _plugin_options: &[PluginOptions],
    _builder: &FrontendBuilder,
) -> Result<()>

Sets up frontend plugins after the FrontendBuilder is constructed but before FrontendBuilder::try_build() and FrontendBuilder::with_plugin().

This is where “feature plugins” are registered — plugins that consume builder context (e.g., KvBackendRef, CatalogManagerRef) to construct themselves.