fix!: BREAKING CHANGE: vector_store: search API response fix (#4080)

# What does this PR do?
- search_query in the vector store search API should be a list,
according to https://github.com/openai/openai-openapi


## Test Plan
modified tests


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* #4086
* __->__ #4080

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

@@ -637,7 +637,7 @@ class OpenAIVectorStoreMixin(ABC):
                     break
 
             return VectorStoreSearchResponsePage(
-                search_query=search_query,
+                search_query=query if isinstance(query, list) else [query],
                 data=data,
                 has_more=False,  # For simplicity, we don't implement pagination here
                 next_page=None,
@@ -647,7 +647,7 @@ class OpenAIVectorStoreMixin(ABC):
             logger.error(f"Error searching vector store {vector_store_id}: {e}")
             # Return empty results on error
             return VectorStoreSearchResponsePage(
-                search_query=search_query,
+                search_query=query if isinstance(query, list) else [query],
                 data=[],
                 has_more=False,
                 next_page=None,