fix: prefetch first item when selection == 'manual' | 'auto_insert'

Related to #627

lua/blink/cmp/completion/init.lua

@@ -78,7 +78,9 @@ function completion.setup()
   -- run 'resolve' on the item ahead of time to avoid delays
   -- when accepting the item or showing documentation
   list.select_emitter:on(function(event)
-    if event.item == nil then return end
+    -- when selection == 'manual' | 'auto_insert', we still want to prefetch the first item
+    local item = event.item or list.items[1]
+    if item == nil then return end
     require('blink.cmp.completion.prefetch')(event.context, event.item)
   end)
 end