Fix off-by-one in split_chunk upper bound - #10342
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split_chunk() splits a chunk [start, end) into [start, split_at) and [split_at, end). Per the invariant documented in the code, both resulting ranges must have length at least 1, so the valid split points are [start + 1, end - 1]. The lower bound was correct, but the upper bound required the right range to have length at least 2. This rejected split_at = end - 1 (a valid length-1 right range) and made a chunk of width 2 unsplittable via an explicit split_at, even though the default midpoint path picks and accepts that same point for a width-2 chunk. Allow split points up to end - 1 so the bounds are symmetric and match the documented invariant. Empty-range splits remain rejected.
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split_chunk() splits a chunk [start, end) into [start, split_at) and [split_at, end). Per the invariant documented in the code, both resulting ranges must have length at least 1, so the valid split points are [start + 1, end - 1].
The lower bound was correct, but the upper bound required the right range to have length at least 2. This rejected split_at = end - 1 (a valid length-1 right range) and made a chunk of width 2 unsplittable via an explicit split_at, even though the default midpoint path picks and accepts that same point for a width-2 chunk.
Allow split points up to end - 1 so the bounds are symmetric and match the documented invariant. Empty-range splits remain rejected.