Function handle_create_tab 

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

Phase E.2b — create a tab inside a workspace. Validates the parent exists; otherwise emits Event::Error (non-fatal). On success: assigns a UUID, appends to the workspace’s tab_ids, inserts into state.tabs, emits Event::TabCreated. If the workspace had no active tab, the new tab also becomes active and an Event::ActiveTabChanged is emitted alongside.

NOT idempotent on retry (same UUID-assignment caveat as handle_create_workspace).