feat(misc): add message batches API notebook

Covers the Anthropic Batch API end-to-end: submitting async batches of
requests at 50% cost savings, polling for completion, streaming results
via client.beta.messages.batches.results(), per-request outcome handling
(succeeded/errored/canceled/expired), cost comparison math, and error
retry patterns. Uses claude-haiku-4-5.

Author: RudrenduPaul