In BF-30376, we saw that the collection_defragmentation test could, in very rare occasions, send a merge chunks command where the min bound was greater than the max bound. This wouldn't be a problem in production because this range wouldn't pass the checks we have that the range to be merged matches existing chunks.
However, in tests, this causes a dassert to fail when we create a chunk range out of these bounds values.

This can be fixed simply by checking in the shardsvrMergeChunks command if the max bound is less than the min bound, and throwing an error if so.