Function handle_set_active_tab 

pub(super) fn handle_set_active_tab(
    state: &mut State,
    workspace_id: String,
    tab_id: String,
) -> Vec<Event>

Phase E.2b — set a workspace’s active tab. No-op if already active. Errors (non-fatal) if the workspace doesn’t exist or the tab isn’t in that workspace’s tab list.