fix: convert BigInt to strings in Arrow IPC row.toJSON()

[jamesbroadhead]

Summary

• row.toJSON() from apache-arrow preserves BigInt for Int64 columns. When this data is later serialized with JSON.stringify() for SSE streaming, it throws TypeError: Do not know how to serialize a BigInt.
• Convert BigInt values to strings in _transformArrowAttachment during row transformation, before the data reaches JSON serialization.
• This ensures Arrow IPC deserialization works correctly for serverless warehouses that return inline results as Arrow IPC in result.attachment.

Test plan

• Verify that queries returning Int64 columns via Arrow IPC attachment no longer throw TypeError: Do not know how to serialize a BigInt during SSE streaming
• Verify that non-BigInt columns are unaffected by the change
• Verify that the string representation of BigInt values is correct and usable downstream

This pull request was AI-assisted by Isaac.

[jamesbroadhead]

Superseded by #315 — recreated against the upstream repo so CI workflows can run (fork PRs don't get secrets, hence the broken Lint/Unit/Playground/Docs checks here).