New task: MessageBatch (create / poll / retrieve / cancel)

Part of #75.

## What

Add a `MessageBatch` task to submit, poll, and retrieve Anthropic Message Batches — the async batch API that delivers 50% cost savings. This is a natural fit for Kestra orchestration: create the batch, wait (with a polling sub-flow or a `WaitFor`), then retrieve results.

## Suggested design

Three modes (or three separate tasks):

- **`CreateBatch`**: accepts a list of `BatchRequest` (custom_id + `ChatCompletion`-like params); returns `batchId`.
- **`RetrieveBatch`** / **`PollBatch`**: takes `batchId`; returns status (`in_progress` | `ended`) and, when ended, the result file URL or a list of per-request outputs.
- **`CancelBatch`**: takes `batchId`.

Alternatively, a single `MessageBatch` task with a `mode` enum property.

## Output

- `batchId`, `status`, `requestCounts` (processing / succeeded / errored / canceled / expired).
- When `mode = retrieve` and status is `ended`: `results` list mirroring `ChatCompletion.Output` per request.

## References

- Anthropic Batch API docs
- SDK: `client.messages().batches().*`

Start with the Anthropic Batch API documentation and the SDK entry points named in the issue: client.messages().batches().*. Define the create, poll or retrieve, and cancel scope, then verify that the outputs cover batchId, status, requestCounts, and ended-batch results.

Written by the indexing model from the issue text.