[issues/306] Fix toInputSelection zero-width selection bug

[couimet]

Summary

Fixes SELECTION_ZERO_WIDTH validation error when using Ctrl+L or triple-click to select full lines. When VSCode reports a selection ending at the start of the next line (e.g., {line: 20, char: 0} to {line: 21, char: 0}), the normalization now correctly sets the end character to the line length instead of 0.

Changes

• Add adjustedEndCharacter calculation in toInputSelection.ts when normalizing trailing newline selections
• Update 4 tests that previously codified the buggy behavior (expected character: 0 instead of line length)
• Add 3 integration tests in generateLinkFromSelections.test.ts that verify full flow without mocking

Key Discoveries

• The bug triggers when sel.end.line > sel.start.line && sel.end.character === 0 (the includesTrailingNewline condition)
• Affects both single-line (Ctrl+L) and multi-line (multiple Ctrl+L) selections
• Character positions are ignored when coverage = FullLine, so the fix only prevents validation failure - link output (#L5, #L5-L10) remains unchanged
• Issue Fix stale selection error after external file modification #260 (external file modification causing stale selections) was closed as won't-do since the logs revealed this different root cause

Test Plan

• All 1403 tests pass
• 4 test expectations updated to expect correct line lengths
• 3 integration tests added to verify link format preservation:
  • Single full-line selection (Ctrl+L) → #L5 (no char positions)
  • Multi-line full selection (Ctrl+L x3) → #L5-L7 (no char positions)
  • Partial selection ending at next line → #L5C9-L5C19 (with char positions)
• Manual testing: Open any file, Ctrl+L to select a line, generate link - should work without error

Documentation

• CHANGELOG.md: Entry added to Fixed section
• README.md: Not needed - internal bug fix, no user-facing API changes

Related

• Closes Fix toInputSelection incorrectly normalizing full-line selections to zero-width #306
• Related to Stale selection causes SELECTION_ZERO_WIDTH error when file is modified externally #258 (parent issue for stale selection debugging)
• Closes Fix stale selection error after external file modification #260 as won't-do (original hypothesis was wrong)

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch issues/306

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