fix(plugins): find the new copy via _existing_install, as install_plugin does

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/plugin_system/store_install.py

@@ -95,7 +95,7 @@ class _InstallMixin:
             if installed:
                 self._discard_backup(
                     plugin_id, backup_path, "install",
-                    new_path=self._find_plugin_path(plugin_id) or plugin_path)
+                    new_path=self._existing_install(plugin_id) or plugin_path)
                 return True
 
             self._restore_backup(plugin_id, plugin_path, backup_path, "Install")

src/plugin_system/store_update.py

@@ -309,7 +309,7 @@ class _UpdateMixin:
                 # rather than the old directory name.
                 self._discard_backup(
                     plugin_id, backup_path, "update",
-                    new_path=self._find_plugin_path(plugin_id) or plugin_path)
+                    new_path=self._existing_install(plugin_id) or plugin_path)
                 return True
 
             # Bad network, registry error...: the user keeps a working plugin.