[Trivikram Kamat (trivikr)]

FullTextDocument.update clamped startLine/endLine to 0 but not to the last line. An incremental change starting past the end of the document could leave holes in the line offsets, so lineCount came out wrong and positionAt/offsetAt returned NaN/undefined.

const d = TextDocument.create('file:///a', 'txt', 0, 'a\nb');
TextDocument.update(d, [{ range: { start: { line: 5, character: 0 }, end: { line: 6, character: 0 } }, text: 'x\n' }], 1);
d.lineCount; // 7, expected 3

This PR clamps both lines to the last line index, the same way offsetAt handles out-of-range positions, and adds a regression test.

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🟢 Approval recommended

No active review issues are recorded for the change addressing incremental updates beyond the document’s end.

No open issues 🎉

_{Review effort: Balanced}