feat: Add search_mode support to OpenAI vector store API (#2500)

# What does this PR do?
Add search_mode parameter (vector/keyword/hybrid) to
openai_search_vector_store method. Fixes OpenAPI
code generation by using str instead of Literal type.

Closes: #2459 

## Test Plan
<!-- Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
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Signed-off-by: Varsha Prasad Narsing <varshaprasad96@gmail.com>

llama_stack/apis/vector_io/vector_io.py

@@ -372,6 +372,7 @@ class VectorIO(Protocol):
         max_num_results: int | None = 10,
         ranking_options: SearchRankingOptions | None = None,
         rewrite_query: bool | None = False,
+        search_mode: str | None = "vector",  # Using str instead of Literal due to OpenAPI schema generator limitations
     ) -> VectorStoreSearchResponsePage:
         """Search for chunks in a vector store.
 
@@ -383,6 +384,7 @@ class VectorIO(Protocol):
         :param max_num_results: Maximum number of results to return (1 to 50 inclusive, default 10).
         :param ranking_options: Ranking options for fine-tuning the search results.
         :param rewrite_query: Whether to rewrite the natural language query for vector search (default false)
+        :param search_mode: The search mode to use - "keyword", "vector", or "hybrid" (default "vector")
         :returns: A VectorStoreSearchResponse containing the search results.
         """
         ...