fix(file_selector): make sure to flatten selected_paths if picker yields a single string (#1153)

lua/avante/config.lua

@@ -252,6 +252,8 @@ M._defaults = {
     provider = "native",
     -- Options override for custom providers
     provider_opts = {},
+    --- @type fun(selected_paths: string[] | string | nil): nil
+    handler = nil,
   },
   suggestion = {
     debounce = 600,