[ty_test] Store HoverAssertion.column as zero-based
Change the column field in HoverAssertion from OneIndexed to usize,
storing it as a zero-based index. This matches how it's used and
eliminates unnecessary conversions between zero-based and one-based
indexing.
The arrow position from find() is already zero-based, and TextSize
uses zero-based offsets, so this is more natural.
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Co-Authored-By: Claude <noreply@anthropic.com>
