Function read_group_metadata_from_dict 

pub fn read_group_metadata_from_dict(
    mls_group: &MlsGroup,
) -> Result<Option<GroupMetadataReturn>, ComponentSourceError>

Synthesize a [GroupMetadata] from the AppData dictionary. Returns Ok(None) if the critical immutable seeds are absent.

Encoding mirrors the sender-side synthesis in crate::app_data::migration::synthesize_canonical_subset_for_validation:

• CONVERSATION_TYPE: 4 big-endian bytes of ConversationType as i32 (see encode_conversation_type there).
• CREATOR_INBOX_ID: the versioned InboxId TLS wire form (varint(version) || 32-byte payload) — the same shape every other inbox-id-bearing component on the new path uses. Reader hex-encodes the decoded id back into the legacy GroupMetadata::creator_inbox_id: String slot.
• DM_MEMBERS: TlsSet<InboxId> with exactly two elements — matches the declared ComponentType::TlsSetInboxId and the sender’s encode_dm_members. The writer rejects self-DMs (identical slots) up front; readers that see a 1-element set surface MalformedComponentValue.
• ONESHOT_MESSAGE: prost-encoded OneshotMessage.