Fix server panic when undoing an edit

[MichaReiser]

Summary

Fixes #13674 and possibly astral-sh/ruff-vscode#607

The TextDocument::apply_changes contained some naive logic to reuse the LineIndex if it happened that the content didn't change.
Hower, that logic was incorrect because it compared the new_content with the original document content rather than the content
from the last iteration. This resulted in a stale LineIndex when an edit reverts the content to the original content.

I decided to remove the optimization alltogether because it's too naive to be useful in practice. We could apply a logic similar
to Biome's, we avoid recomputing the LineIndex when the previous edits only changed lines further down in the document. source
However, I don't think that this is a big perf bottlneck. Computing a LineIndex is relatively cheap.

Test Plan

I added a unit test

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[dhruvmanila]

Nice! Thanks for looking into it