Problem

replicate.run() ignores AbortSignal in two ways:

1. Pre-aborted signal: If signal.aborted is already true before calling run(), the SDK starts the prediction anyway instead of throwing immediately.

2. Mid-poll abort: If the signal becomes aborted during polling, run() stops polling and cancels the prediction, but then silently returns instead of throwing AbortError. The caller has no way to distinguish between a successful completion and a cancelled request.

Fix

Before starting: Check signal.aborted and throw AbortError immediately if already aborted.

After cancellation: Throw AbortError instead of returning the prediction output. The predictions.cancel() call is wrapped in .catch() to handle cases where the prediction already completed on Replicate's servers.

// Pre-aborted check
if (signal?.aborted) {
  const error = new Error("Aborted");
  error.name = "AbortError";
  throw error;
}

// Post-cancellation throw (instead of silent return)
if (signal && signal.aborted) {
  await this.predictions.cancel(prediction.id).catch(() => {});
  const error = new Error("Aborted");
  error.name = "AbortError";
  throw error;
}

This matches how fetch() handles AbortSignal and allows callers to properly distinguish cancelled requests from completed ones.

Fixes #370