Fix integer overflow in ParallelHelper.For range computation

[sayantanmandal1]

Fixes #1188. ParallelHelper.For computed the range size with Math.Abs(start - end) in 32-bit arithmetic, which overflows for ranges larger than int.MaxValue: the subtraction wraps around (silently running single-threaded) and Math.Abs(int.MinValue) throws OverflowException. The range and batch sizes are now computed with 64-bit arithmetic; the per-iteration indices stay within [start, end] so the narrowing casts are safe. Added deterministic regression tests.

Closes #1188

[sayantanmandal1]

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