Function apply_and_highlight 

pub async fn apply_and_highlight(
    handle: Arc<dyn Document>,
    text: String,
    spans: Vec<Vec<StyledSpan>>,
    pending_highlights: Option<PendingSyntheticHighlightsHandle>,
    buffer_id: BufferId,
)

Apply a full buffer replacement, then store highlights and fire the waker so the Editor repaints immediately. Async — call from a tokio task (NOT spawn_blocking). The blocking I/O phase (build_and_format) must complete before calling this.