Function default_branding_ref 

pub fn default_branding_ref(
    ext: &Option<Extension<Arc<DefaultBranding>>>,
) -> Option<&BrandingConfig>

Flatten the embedder default-branding extension into the plain reference form handlers need to feed into resolve_branding.

Handlers declare the extractor as Option<Extension<Arc<DefaultBranding>>>; they all then need the inner &BrandingConfig. This helper removes the per-site as_ref().map(|Extension(d)| &d.0) boilerplate.