Document bulk import integration_id option

[compair-steven]

Adds the supported integration_id argument to the bulk import guide. The SDK signature, request body, unit tests, and smoke-test README already cover this option, but the how-to guide currently only documents uri and error_mode.

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Note

Low Risk
Documentation-only change with no runtime or API behavior impact.

Overview
The bulk import how-to guide now documents integration_id on start_import, for projects that use a configured storage integration when reading from S3/GCS/Azure.

A new Use a storage integration section shows passing integration_id alongside uri, and notes it can be combined with error_mode.

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