fix(ui): warn when message for part is missing

Add a warning notification when a part update references a message that
cannot be found in the render state. This helps with debugging and
improves visibility into potential state inconsistencies.

Author: jugarpeupv

lua/opencode/ui/renderer.lua

@@ -731,6 +731,7 @@ function M.on_part_updated(properties, revert_index)
 
   local rendered_message = M._render_state:get_message(part.messageID)
   if not rendered_message or not rendered_message.message then
+    vim.notify('Could not find message for part: ' .. vim.inspect(part), vim.log.levels.WARN)
     return
   end