Function resolve_method_group 

pub fn resolve_method_group(
    method: &MethodInfo,
    registry: &Option<GroupRegistry>,
) -> Result<Option<String>>

Resolve a method’s group against the registry.

When the method has group = "id", the registry must be present and must contain a matching ID — otherwise a compile error is emitted. The returned string is the display name from the registry.

When the method has no group attribute, returns None.