handler-source: return string, let page template handle highlighting

highlight was double-wrapping in (code ...). Match component-source
pattern — return pretty-printed string, page handles display.

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

sx/sx/data/helpers.sx

@@ -57,7 +57,7 @@
                   (append parts (list (make-keyword "returns") returns))
                   parts))
               (parts (append parts (list params body))))
-            (highlight (pretty-print parts))))))))
+            (pretty-print parts)))))))
 
 (define _spec-dirs (list "spec" "web" "shared/sx/ref" "lib"))