fix(vector store)!: fix file content API (#4105)

# What does this PR do?
- changed to match
https://app.stainless.com/api/spec/documented/openai/openapi.documented.yml

## Test Plan
updated test CI

src/llama_stack/providers/utils/memory/openai_vector_store_mixin.py

@@ -30,7 +30,7 @@ from llama_stack.apis.vector_io import (
     VectorStoreContent,
     VectorStoreDeleteResponse,
     VectorStoreFileBatchObject,
-    VectorStoreFileContentsResponse,
+    VectorStoreFileContentResponse,
     VectorStoreFileCounts,
     VectorStoreFileDeleteResponse,
     VectorStoreFileLastError,
@@ -921,22 +921,21 @@ class OpenAIVectorStoreMixin(ABC):
         self,
         vector_store_id: str,
         file_id: str,
-    ) -> VectorStoreFileContentsResponse:
+    ) -> VectorStoreFileContentResponse:
         """Retrieves the contents of a vector store file."""
         if vector_store_id not in self.openai_vector_stores:
             raise VectorStoreNotFoundError(vector_store_id)
 
-        file_info = await self._load_openai_vector_store_file(vector_store_id, file_id)
         dict_chunks = await self._load_openai_vector_store_file_contents(vector_store_id, file_id)
         chunks = [Chunk.model_validate(c) for c in dict_chunks]
         content = []
         for chunk in chunks:
             content.extend(self._chunk_to_vector_store_content(chunk))
-        return VectorStoreFileContentsResponse(
-            file_id=file_id,
-            filename=file_info.get("filename", ""),
-            attributes=file_info.get("attributes", {}),
-            content=content,
+        return VectorStoreFileContentResponse(
+            object="vector_store.file_content.page",
+            data=content,
+            has_more=False,
+            next_page=None,
         )
 
     async def openai_update_vector_store_file(