diagnostics: Always expand at least `multibuffer_context_lines` per diagnostic (#56172)

Otherwise we can sometimes end up with single line excerpts which looks
very off



Release Notes:

- Improved the minimum size of diagnostics pane excerpts
b8c3167a9dLukas Wirth committed on 5/8/2026, 2:18:44 PM· committed by GitHubparent954ac0f
3 files changedLine totals unavailable