gh-89520: IDLE - Fix default value logic in configdialog load_extensions

Fix two issues in load_extensions:

1. def_str (the "default" value) was sourced from user config when the
   option existed there. This meant opening the extensions dialog could
   silently revert user customizations, since the user's value would be
   treated as the default and then removed. Now prefers default config,
   falling back to user config only for user-only extensions.

2. The value lookup was skipping user config for default-only options.
   Restore the original behavior of always trying user config first
   (which returns def_obj as fallback when the option is absent).

Author: gpshead

Lib/idlelib/configdialog.py

@@ -1978,11 +1978,11 @@ def load_extensions(self):
             opt_list = enables + opt_list
 
             for opt_name in opt_list:
-                if opt_name in user:
-                    def_str = self.ext_userCfg.Get(
+                if opt_name in default:
+                    def_str = self.ext_defaultCfg.Get(
                             ext_name, opt_name, raw=True)
                 else:
-                    def_str = self.ext_defaultCfg.Get(
+                    def_str = self.ext_userCfg.Get(
                             ext_name, opt_name, raw=True)
                 try:
                     def_obj = {'True':True, 'False':False}[def_str]
@@ -1995,14 +1995,9 @@ def load_extensions(self):
                         def_obj = def_str
                         opt_type = None
                 try:
-                    if opt_name in user:
-                        value = self.ext_userCfg.Get(
-                                ext_name, opt_name, type=opt_type, raw=True,
-                                default=def_obj)
-                    else:
-                        value = self.ext_defaultCfg.Get(
-                                ext_name, opt_name, type=opt_type, raw=True,
-                                default=def_obj)
+                    value = self.ext_userCfg.Get(
+                            ext_name, opt_name, type=opt_type, raw=True,
+                            default=def_obj)
                 except ValueError:  # Need this until .Get fixed.
                     value = def_obj  # Bad values overwritten by entry.
                 var = StringVar(self)