xtab: fix: clamp tagged patch range to document
Tagged prompt content can add lines beyond the source document. Clamp the CustomDiffPatch pseudo-window to the source range to avoid an unexpected failure.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 62cca64b-0581-410e-ab89-034e20f02ca7