The previous (quantity * 100_000.0) as i64 silently truncated
fractional lots and could overflow on extreme values. Added
convert_quantity_to_volume() that validates the result is finite,
non-negative, and within i64 range, using round() instead of
truncation. Includes 7 unit tests covering normal values, fractional
rounding, overflow, negative, NaN, infinity, and zero edge cases.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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