⚡ Optimize RecordBatchBuilder::push schema keys lookup

[edithatogo]

💡 What: Add a side HashSet for tracking schema keys instead of searching a Vec.
🎯 Why: Resolves O(N) lookup array membership checks during row processing.
📊 Measured Improvement: ~7.8% speedup (-8.28% to -7.48% change on 1000 record pushes).

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PR created automatically by Jules for task 10260900521670163117 started by @edithatogo

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