Function apply_labels_to_number 

pub async fn apply_labels_to_number(
    client: &Octocrab,
    owner: &str,
    repo: &str,
    number: u64,
    labels: &[String],
) -> Result<Vec<String>>

Apply labels to an issue or PR by number.

Simplified label-only application function for PRs (no milestone, no merge logic). Returns an error if the GitHub API call fails.