fix: handle PermissionError for local_folder

[yoomlam]

Ticket

Following up from this comment

Changes

If needed, set alternative /tmp local_folder before the for-loop

Testing

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[Copilot]

Pull request overview

This PR improves the RAG ingestion path by validating write permissions for the local_folder used for S3 downloads and falling back to /tmp when needed, while keeping the top-level local ingestion directory consistent with the prior fix in PR #173.

Changes:

• In populate_vector_db, add a pre-check that attempts to create the configured local_folder, logging and switching to a /tmp/-based path on PermissionError.
• Update download_s3_folder_to_local to accept an explicit local_folder parameter instead of deriving it from the S3 prefix, ensuring all regions download under the same top-level local folder.
• Remove the per-region directory-creation and permission handling from download_s3_folder_to_local, centralizing that logic in populate_vector_db.

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