Function resolve_tab_id_for_block 

pub(super) fn resolve_tab_id_for_block(
    mstore: &Store,
    block_id: &str,
) -> Result<String, String>

Find which tab a given block currently belongs to, by scanning every tab’s blockids. Needed because resolve_tab_id only resolves an explicit tab_id or “the active tab” — neither answers “which tab is MY OWN block in,” which a caller passing its own block id (e.g. an MCP tool’s split_reference_block_id) actually needs.