Fix some RenderShortcodes error cases

This issue fixes two cases where `{{__hugo_ctx` artifacts were left in the rendered output:

1. Inclusion when `.RenderShortcodes` is wrapped in HTML.
2. Inclusion of Markdown file without a trailing newline in some cases.

Closes #12854
Updates #12998

hugolib/shortcode_page.go

@@ -125,3 +125,7 @@ func newPageForRenderHook(p *pageState) page.Page {
 func (p *pageForRenderHooks) Unwrapv() any {
 	return p.p
 }
+
+func (p *pageForRenderHooks) String() string {
+	return p.p.String()
+}