feat: add buffer validation for chat window

Add validate() method to Chat class that ensures the window buffer stays in
sync. This prevents potential issues where the window could show incorrect
buffer content by validating and resetting the buffer if needed.

lua/CopilotChat/chat.lua

@@ -97,6 +97,17 @@ function Chat:create()
   return bufnr
 end
 
+function Chat:validate()
+  Overlay.validate(self)
+  if
+    self.winnr
+    and vim.api.nvim_win_is_valid(self.winnr)
+    and vim.api.nvim_win_get_buf(self.winnr) ~= self.bufnr
+  then
+    vim.api.nvim_win_set_buf(self.winnr, self.bufnr)
+  end
+end
+
 function Chat:visible()
   return self.winnr
     and vim.api.nvim_win_is_valid(self.winnr)