refactor: remove ResponseGuardrail cast by fixing implementation signature

The implementation already handles both str and ResponseGuardrailSpec via
extract_guardrail_ids(), so update the signature to match the API.

src/llama_stack/providers/inline/agents/meta_reference/agents.py

@@ -349,10 +349,6 @@ class MetaReferenceAgentsImpl(Agents):
         guardrails: list[ResponseGuardrail] | None = None,
     ) -> OpenAIResponseObject:
         assert self.openai_responses_impl is not None, "OpenAI responses not initialized"
-        from llama_stack.apis.agents.agents import ResponseGuardrailSpec
-        from typing import cast as typing_cast
-        # Cast guardrails to the more specific type expected by the implementation
-        guardrails_spec = typing_cast(list[ResponseGuardrailSpec] | None, guardrails)
         result = await self.openai_responses_impl.create_openai_response(
             input,
             model,
@@ -367,9 +363,9 @@ class MetaReferenceAgentsImpl(Agents):
             tools,
             include,
             max_infer_iters,
-            guardrails_spec,
+            guardrails,
         )
-        return typing_cast(OpenAIResponseObject, result)
+        return result  # type: ignore[no-any-return]
 
     async def list_openai_responses(
         self,

src/llama_stack/providers/inline/agents/meta_reference/responses/openai_responses.py

@@ -254,7 +254,7 @@ class OpenAIResponsesImpl:
         tools: list[OpenAIResponseInputTool] | None = None,
         include: list[str] | None = None,
         max_infer_iters: int | None = 10,
-        guardrails: list[ResponseGuardrailSpec] | None = None,
+        guardrails: list[str | ResponseGuardrailSpec] | None = None,
     ):
         stream = bool(stream)
         text = OpenAIResponseText(format=OpenAIResponseTextFormat(type="text")) if text is None else text