Function enqueue_pull_request 

pub fn enqueue_pull_request(
    bin: &Path,
    pr_node_id: &str,
) -> Result<EnqueueOutcome>

Enqueues one PR into its repo’s merge queue via the enqueuePullRequest mutation. Blocking — run on a blocking thread.

Unlike [run_gh_graphql], a GraphQL error here is not a hard Err: gh exits non-zero on a rejected mutation, and we map that (and a 200-with-errors) to EnqueueOutcome::Rejected so one un-enqueuable PR lands in the batch’s failed[] list while the rest proceed. Err is reserved for “could not run gh at all” (e.g. the binary is missing).