esutil: wrap bulk item failures with response context

[thani-ath-nain]

Summary

Part of #1428 (PR 2/3).

Adds response context to per-item bulk indexer failures so callers can inspect the failed item details via errors. As, while preserving existing callback flow and behavior.

What changed

• Exported additive error type.
• Updated the bulk flush item-failure path to pass the error into OnFailure.

Compatibility

• No exported API changes.
• No signature changes.
• This is additive: consumers can opt in to richer inspection using errors.As(err, *BulkIndexerItemError).

Testing

make lint && make test-unit

[prodsecmachine]

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