Fix highlight undefined symbol by expanding component strings server-side

Page helpers returning SX component call strings (e.g. "(~docs-intro-content)")
were sent to the client unexpanded. Components containing Python-only helpers
like `highlight` then failed with "Undefined symbol" on the client. Now
async_eval_slot_to_sx re-parses and expands these strings server-side.

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

shared/sx/async_eval.py

@@ -1064,7 +1064,16 @@ async def async_eval_slot_to_sx(
     if result is None or result is NIL:
         return SxExpr("")
     if isinstance(result, str):
-        # Plain strings from page helpers are already sx source — don't quote.
+        # Page helpers return sx source strings.  If the string is a
+        # component call (starts with "(~"), re-parse and expand it
+        # server-side so that Python-only helpers like ``highlight``
+        # inside the component body get evaluated here, not on the client.
+        stripped = result.strip()
+        if stripped.startswith("(~"):
+            from .parser import parse_all
+            parsed = parse_all(stripped)
+            if parsed:
+                return await async_eval_slot_to_sx(parsed[0], env, ctx)
         return SxExpr(result)
     return SxExpr(serialize(result))