[ty_test] Calculate hover column at parse time
Move the column calculation from generate_hover_outputs into
HoverAssertion::from_str() by passing LineIndex and SourceText to
the parse() method.
This simplifies the code and centralizes the column calculation logic
in one place during parsing, rather than spreading it across multiple
locations. The HoverAssertion now directly stores the final column
position in the line, ready to use.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
