Fix cursor position error when rendering empty chat buffer

[Copilot]

The chat UI crashes with "Cursor position outside buffer" when starting a new session because nvim_win_set_cursor is called with line 0 on an empty buffer.

Changes

• Add bounds check before setting cursor position in render() to prevent invalid cursor position when buffer is empty
• Only call nvim_win_set_cursor when #lines > 0

-- Before
if vim.api.nvim_win_is_valid(M.state.winid) then
  vim.api.nvim_win_set_cursor(M.state.winid, { #lines, 0 })
end

-- After
if vim.api.nvim_win_is_valid(M.state.winid) and #lines > 0 then
  vim.api.nvim_win_set_cursor(M.state.winid, { #lines, 0 })
end

This occurs in new_session() which clears all messages before the backend creates a new session, resulting in a 0-line buffer render.

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