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>

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) {