Fix multiline error markers and add column validation tests
Improvements:
- Multiline errors now show markers on both first and last lines
- First line: marker at start_column
- Last line: marker at end_column with message
- Middle lines: no marker (just > indicator)
- Messages only render once on the last error line's marker
- Column clamping allows positions one past line length (standard behavior)
- Single-line errors with end_column show spanning markers (^^^)
New test coverage:
- test_multiline_error_with_message: validates message placement
- test_invalid_column_start_out_of_bounds: column past line end
- test_invalid_column_end_before_start: reversed columns
- test_invalid_column_both_out_of_bounds: both columns invalid
- test_invalid_multiline_end_column_out_of_bounds: multiline clamping
All 20 tests passing.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
