Fix RenderShortcodes leaking context markers when indented

Strip leading whitespace from Hugo context marker lines before
Goldmark parsing to prevent them from being treated as indented
code blocks.

Fixes #12457

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hugolib/page__content.go

@@ -695,6 +695,8 @@ func (c *cachedContentScope) contentToC(ctx context.Context) (contentTableOfCont
 			return nil, err
 		}
 
+		ct.contentToRender = hugocontext.DedentMarkers(ct.contentToRender)
+
 		if hasVariants {
 			p.incrPageOutputTemplateVariation()
 		}

hugolib/rendershortcodes_test.go

@@ -552,3 +552,32 @@ title: "includeme"
 
 	b.AssertNoRenderShortcodesArtifacts()
 }
+
+// Issue 12457.
+func TestRenderShortcodesCodeBlock(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+disableKinds = ['section','rss','sitemap','taxonomy','term']
+-- layouts/_shortcodes/foo.html --
+{{ $p := site.GetPage "includeme" }}
+    {{ $p.RenderShortcodes }}
+-- layouts/home.html --
+{{ .Content }}
+-- content/_index.md --
+---
+title: home
+---
+{{% foo %}}
+-- content/includeme.md --
+---
+title: "includeme"
+---
+Some markdown.
+    `
+
+	b := Test(t, files)
+	b.AssertNoRenderShortcodesArtifacts()
+	b.AssertFileContentEquals("public/index.html", "<p>Some markdown.</p>\n")
+}

markup/goldmark/hugocontext/hugocontext.go

@@ -102,8 +102,18 @@ var (
 	hugoCtxEndDelim     = []byte("}}")
 	hugoCtxClosingDelim = []byte("/}}")
 	hugoCtxRe           = regexp.MustCompile(`{{__hugo_ctx( pid=\d+)?/?}}\n?`)
+	hugoCtxIndentedRe   = regexp.MustCompile(`(?m)^[ \t]+({{__hugo_ctx[^\n]*}})`)
 )
 
+// DedentMarkers removes leading whitespace from Hugo context marker lines
+// to prevent them from being treated as indented code blocks by Goldmark.
+func DedentMarkers(b []byte) []byte {
+	if !bytes.Contains(b, hugoCtxPrefix) {
+		return b
+	}
+	return hugoCtxIndentedRe.ReplaceAll(b, []byte("$1"))
+}
+
 // Strip strips any Hugo context markers from b.
 func Strip(b []byte) []byte {
 	if !bytes.Contains(b, hugoCtxPrefix) {