[ty_test] Fix column units: use character offset not byte offset
The column field was being treated inconsistently - calculated as a
character offset but used as a byte offset. This would break on any
line with multi-byte UTF-8 characters before the hover position.
Fixes:
1. Use chars().position() instead of find() to get character offset
2. Use LineIndex::offset() with PositionEncoding::Utf32 to properly
convert character offset to byte offset (TextSize)
3. Document that column is a UTF-32 character offset
This ensures hover assertions work correctly with Unicode text.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
