Submit and reconcile offline message batches

Goal

Goal: Submit many independent prompts asynchronously, then reconcile successes and failures by `custom_id`.

Steps

1. Assign each request a stable custom_id and build a {"custom_id":..., "params":{...}} request. [batch]
2. Submit with client.beta.messages.batches.create(requests=...), persist the batch ID, and poll retrieve(batch_id) until processing_status == "ended". [batch]
3. Read result items into a result store using idempotent upserts by custom_id; separate success, error, and missing items. [batch]

How to verify

The request count reconciles to success, failure, and missing items; retry only failed items and retain original batch mapping.

Troubleshooting

If a batch stalls, poll with bounded backoff and retain status; creation success does not mean every item completed.