Function extract_legacy_group_mutable_metadata 

pub fn extract_legacy_group_mutable_metadata(
    group: &MlsGroup,
) -> Result<GroupMutableMetadata, GroupMutableMetadataError>

Read GroupMutableMetadata from the legacy group-context extension only.

Use only when the caller is certain the group is unmigrated — on post-bootstrap groups the legacy extension is gone and this returns GroupMutableMetadataError::MissingExtension.

For capability-aware reads that handle both legacy and migrated groups, use extract_group_mutable_metadata_capability_aware in the xmtp_mls crate at xmtp_mls::groups::app_data::component_source. (xmtp_mls_common cannot rustdoc-link to it because the dependency direction is one-way — this comment is the pointer.)