feat: add responses input items api (#2239)

# What does this PR do?
TSIA

## Test Plan
added integration and unit tests

llama_stack/apis/agents/agents.py

@@ -31,6 +31,7 @@ from llama_stack.apis.tools import ToolDef
 from llama_stack.schema_utils import json_schema_type, register_schema, webmethod
 
 from .openai_responses import (
+    ListOpenAIResponseInputItem,
     ListOpenAIResponseObject,
     OpenAIResponseInput,
     OpenAIResponseInputTool,
@@ -630,3 +631,25 @@ class Agents(Protocol):
         :returns: A ListOpenAIResponseObject.
         """
         ...
+
+    @webmethod(route="/openai/v1/responses/{response_id}/input_items", method="GET")
+    async def list_openai_response_input_items(
+        self,
+        response_id: str,
+        after: str | None = None,
+        before: str | None = None,
+        include: list[str] | None = None,
+        limit: int | None = 20,
+        order: Order | None = Order.desc,
+    ) -> ListOpenAIResponseInputItem:
+        """List input items for a given OpenAI response.
+
+        :param response_id: The ID of the response to retrieve input items for.
+        :param after: An item ID to list items after, used for pagination.
+        :param before: An item ID to list items before, used for pagination.
+        :param include: Additional fields to include in the response.
+        :param limit: A limit on the number of objects to be returned. Limit can range between 1 and 100, and the default is 20.
+        :param order: The order to return the input items in. Default is desc.
+        :returns: An ListOpenAIResponseInputItem.
+        """
+        ...

llama_stack/apis/agents/openai_responses.py

@@ -216,7 +216,7 @@ OpenAIResponseInputTool = Annotated[
 register_schema(OpenAIResponseInputTool, name="OpenAIResponseInputTool")
 
 
-class OpenAIResponseInputItemList(BaseModel):
+class ListOpenAIResponseInputItem(BaseModel):
     data: list[OpenAIResponseInput]
     object: Literal["list"] = "list"