Fix highlight undefined symbol by expanding component results server-side

When defpage content expressions use case/if branches that resolve to
component calls (e.g. `(case slug "intro" (~docs-intro-content) ...)`),
_aser serializes them for the client. Components containing Python-only
helpers like `highlight` then fail with "Undefined symbol" on the client.

Add _maybe_expand_component_result() which detects when the evaluated
result (SxExpr or string) is a component call starting with "(~" and
re-parses + expands it through async_eval_slot_to_sx server-side.

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

shared/sx/async_eval.py

@@ -1018,6 +1018,31 @@ async def async_eval_to_sx(
     return SxExpr(serialize(result))
 
 
+async def _maybe_expand_component_result(
+    result: Any,
+    env: dict[str, Any],
+    ctx: RequestContext,
+) -> Any:
+    """If *result* is a component call (SxExpr or string starting with
+    ``(~``), re-parse and expand it server-side.
+
+    This ensures Python-only helpers (e.g. ``highlight``) inside the
+    component body are evaluated on the server rather than being
+    serialized for the client where they don't exist.
+    """
+    raw = None
+    if isinstance(result, SxExpr):
+        raw = str(result).strip()
+    elif isinstance(result, str):
+        raw = result.strip()
+    if raw and raw.startswith("(~"):
+        from .parser import parse_all
+        parsed = parse_all(raw)
+        if parsed:
+            return await async_eval_slot_to_sx(parsed[0], env, ctx)
+    return result
+
+
 async def async_eval_slot_to_sx(
     expr: Any,
     env: dict[str, Any],
@@ -1059,21 +1084,16 @@ async def async_eval_slot_to_sx(
                 )
     # Fall back to normal async_eval_to_sx
     result = await _aser(expr, env, ctx)
+    # If the result is a component call (from case/if/let branches or
+    # page helpers returning strings), re-parse and expand it server-side
+    # so that Python-only helpers like ``highlight`` in the component body
+    # get evaluated here, not on the client.
+    result = await _maybe_expand_component_result(result, env, ctx)
     if isinstance(result, SxExpr):
         return result
     if result is None or result is NIL:
         return SxExpr("")
     if isinstance(result, str):
-        # 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))