⚡ Bolt: Reduce allocations by pre-sizing List in CsvLineSplitter

[google-labs-jules]

💡 What: Update CsvLineSplitter to accept an initial capacity for the List used to store split values, and update CsvReader to pass the header count (if available) as the capacity.
🎯 Why: CsvLineSplitter creates a new List for every line. The default capacity is small (4), causing multiple resizes and array allocations for every line if the CSV has many columns (e.g., 50 columns -> 4 resizes).
📊 Impact: Reduces memory allocations and CPU overhead associated with List resizing for every row in the CSV file.
🔬 Measurement: Verified by running dotnet test. Performance improvement is expected in high-throughput parsing scenarios with many columns.

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PR created automatically by Jules for task 466527172311085013 started by @stevehansen

[google-labs-jules]

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