perf(chat): simplify last line/column calculation (#1402)

Replaces Chat:last() method with a standalone last() function for
retrieving the last line and column of the chat buffer. Updates
references to use the new function and removes redundant logic.
This improves code clarity and maintainability.

Author: deathbeam

lua/CopilotChat/ui/chat.lua

@@ -52,6 +52,21 @@ local function match_block_header(header)
   end
 end
 
+--- Get the last line and column of the chat window.
+---@param bufnr number
+---@return number, number
+---@protected
+local function last(bufnr)
+  local line_count = vim.api.nvim_buf_line_count(bufnr)
+  if line_count == 0 then
+    return 0, 0
+  end
+  local last_line = line_count - 1
+  local last_line_content = vim.api.nvim_buf_get_lines(bufnr, last_line, last_line + 1, false)
+  local last_column = last_line_content[1] and #last_line_content[1] or 0
+  return last_line, last_column
+end
+
 ---@class CopilotChat.ui.chat.Header
 ---@field filename string
 ---@field start_line number
@@ -426,11 +441,7 @@ function Chat:follow()
     return
   end
 
-  local last_line, last_column, line_count = self:last()
-  if line_count == 0 then
-    return
-  end
-
+  local last_line, last_column = last(self.bufnr)
   vim.api.nvim_win_set_cursor(self.winnr, { last_line + 1, last_column })
 end
 
@@ -557,7 +568,7 @@ function Chat:append(str)
     should_follow_cursor = current_pos[1] >= line_count - 1
   end
 
-  local last_line, last_column, _ = self:last()
+  local last_line, last_column, _ = last(self.bufnr)
 
   local modifiable = vim.bo[self.bufnr].modifiable
   vim.bo[self.bufnr].modifiable = true
@@ -884,22 +895,4 @@ function Chat:render()
   end
 end
 
---- Get the last line and column of the chat window.
----@return number, number, number
----@protected
-function Chat:last()
-  self:validate()
-  local line_count = vim.api.nvim_buf_line_count(self.bufnr)
-  local last_line = line_count - 1
-  if last_line < 0 then
-    return 0, 0, line_count
-  end
-  local last_line_content = vim.api.nvim_buf_get_lines(self.bufnr, -2, -1, false)
-  if not last_line_content or #last_line_content == 0 then
-    return last_line, 0, line_count
-  end
-  local last_column = #last_line_content[1]
-  return last_line, last_column, line_count
-end
-
 return Chat