fix(guardrails): put the cast-ok marker on the cast's own line

The type-discipline gate then failed where the basedpyright one had passed:
LIT006 wants `# cast-ok: <reason>` on the cast call's first line, and my
reason sat four lines above it, so LIT013 also fired -- a marker that
suppresses nothing. Matches the existing cast in this file, which puts the
marker as a trailing comment on `cast(`.

Verified with the repo's own scripts/check_type_discipline.py against the
gate's base SHA: LIT006 and LIT013 are back to the base count and LIT011 is
one lower, so no rule grows. 39 tests pass; ruff clean.

litellm/proxy/guardrails/guardrail_hooks/unified_guardrail/unified_guardrail.py

@@ -245,11 +245,9 @@ def _ensure_litellm_metadata(data: dict, user_api_key_dict: UserAPIKeyAuth) -> N
     )
 
     BaseTranslation.merge_user_api_key_metadata_into_request(
-        # cast-ok: `data` is the request payload, annotated here as a bare
-        # dict, so passing it on is an unknown argument type. _RequestData is
-        # this module's own alias for it and keeps the argument fully known
-        # without widening three signatures or naming Any again.
-        cast(_RequestData, data),
+        cast(  # cast-ok: _RequestData is this module's own alias for the bare-dict request payload
+            _RequestData, data
+        ),
         user_api_key_dict,
     )