chore: converge to upstream (deepkeep feature merged as #33844)

Drop the two remaining local deltas now that the DeepKeep guardrail is
merged upstream and hardened there:
- proxy_server.py normalize_callback string-filter (not taken upstream;
  belongs in a separate PR if still desired)
- add_guardrail_form.tsx preserve={true} (redundant; antd v5 default)

The structured_messages fix made via the PR UI is already present in
upstream's version, so the branch is now identical to
upstream/litellm_internal_staging.

litellm/proxy/proxy_server.py

@@ -15865,17 +15865,13 @@ async def get_config(
         _failure_callbacks = _litellm_settings.get("failure_callback", [])
         _success_and_failure_callbacks = _litellm_settings.get("callbacks", [])
 
-        # Normalize string callbacks to lists; filter out non-string entries
-        # (e.g. callback objects stored in litellm.callbacks at runtime) so
-        # that process_callback() always receives plain string names.
+        # Normalize string callbacks to lists
         def normalize_callback(callback):
             if isinstance(callback, str):
                 return [callback]
             elif callback is None:
                 return []
-            elif isinstance(callback, list):
-                return [c for c in callback if isinstance(c, str)]
-            return []
+            return callback
 
         _success_callbacks = normalize_callback(_success_callbacks)
         _failure_callbacks = normalize_callback(_failure_callbacks)

ui/litellm-dashboard/src/app/(dashboard)/guardrails/_components/add_guardrail_form.tsx

@@ -1064,7 +1064,6 @@ const AddGuardrailForm: React.FC<AddGuardrailFormProps> = ({ visible, onClose, a
           <Form
             form={form}
             layout="vertical"
-            preserve={true}
             initialValues={{
               mode: "pre_call",
               default_on: false,