server: fix edit_file crash on append at end of file (line_start -1) (#24893)

line_start -1 normalized to n+1, so append inserted at lines.begin() + n + 1, one past end() -> heap-buffer-overflow in vector::_M_range_insert.

Normalize -1 to n (insert at end()), restrict -1 to append mode and reject it for replace/delete instead of silently clobbering the last line. Parenthesize the insert offset so empty-file append computes the position as int first, avoiding a transient begin() - 1 on a null vector data pointer.