docs(fix[widgets]): update widget template code reference

why: The template comment should point at the canonical docs._ext.widgets module path.
what:
- update the widget.html comment to reference docs._ext.widgets._base

Author: tony

docs/_widgets/mcp-install/widget.html

@@ -4,7 +4,8 @@
   MCPInstallWidget.context() / the directive's option merge.
 
   Each code block runs through the `highlight` filter (defined in
-  widgets._base.make_highlight_filter) which wraps Sphinx's PygmentsBridge —
+  docs._ext.widgets._base.make_highlight_filter) which wraps Sphinx's
+  PygmentsBridge —
   so the output is byte-identical to a native ``.. code-block::`` block,
   meaning sphinx-copybutton + its prompt-strip regex work automatically.
 #}