Address PR #1219 code review feedback

- Filter undefined model hint names in SamplingRequestPanel, matching
  the pattern already used in InlineSamplingRequest
- Narrow ElicitationFormPanel to accept ElicitRequestFormParams instead
  of the full ElicitRequest['params'] union; removes the 'requestedSchema
  in request' guard and renders the schema unconditionally
- Add isUrlMode type guard in InlineElicitationRequest to properly
  narrow the URL variant; eliminates the 'as { url: string }' cast

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

clients/web/src/components/groups/SamplingRequestPanel/SamplingRequestPanel.tsx

@@ -72,7 +72,9 @@ export function SamplingRequestPanel({
     includeContext,
   } = request;
 
-  const hints = modelPreferences?.hints?.map((h) => h.name) ?? [];
+  const hints =
+    (modelPreferences?.hints?.map((h) => h.name).filter(Boolean) as string[]) ??
+    [];
 
   return (
     <Stack gap="md">