Function resolve_method_group 

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

Resolve method groups against an optional registry.

When a registry is present, each method’s group value must match a declared ID — otherwise a compile error is emitted. The returned string is the display name from the registry.

When no registry is present, method group values are used as literal display names (Tier 1 mode).