fix: close BigQuery client in fileIterator.Close to prevent connection leak

[nishantmonu51]

• QueryAsFiles opens a *bigquery.Client (with a Storage Read gRPC client) and hands it to the returned fileIterator, but the iterator's Close() only removed the temp dir; the client was closed only on error paths, so every successful ingest leaked the client and its gRPC connections/goroutines.
• Close the client in fileIterator.Close(), matching how the Snowflake and Databricks iterators release their connections.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[nishantmonu51]

@codex: review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🚀

Reviewed commit: 198f317222

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