Fix page helper results being quoted as string literals in defpage slots

Page helpers return SX source strings from render_to_sx(), but _aser's
serialize() was wrapping them in double quotes. In async_eval_slot_to_sx,
pass string results through directly since they're already SX source.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/async_eval.py

@@ -943,6 +943,10 @@ async def async_eval_slot_to_sx(
         return result.source
     if result is None or result is NIL:
         return ""
+    # Page helpers return SX source strings from render_to_sx() —
+    # pass through directly instead of quoting via serialize().
+    if isinstance(result, str):
+        return result
     return serialize(result)