feat: add input validation for search mode of rag query

docs/_static/llama-stack-spec.yaml

@@ -10260,7 +10260,8 @@ components:
             content string), {metadata} (chunk metadata dict). Default: "Result {index}\nContent:
             {chunk.content}\nMetadata: {metadata}\n"
         mode:
-          type: string
+          $ref: '#/components/schemas/RAGSearchMode'
+          default: vector
           description: >-
             Search mode for retrieval—either "vector", "keyword", or "hybrid". Default
             "vector".
@@ -10287,6 +10288,17 @@ components:
         mapping:
           default: '#/components/schemas/DefaultRAGQueryGeneratorConfig'
           llm: '#/components/schemas/LLMRAGQueryGeneratorConfig'
+    RAGSearchMode:
+      type: string
+      enum:
+        - vector
+        - keyword
+        - hybrid
+      title: RAGSearchMode
+      description: >-
+        Search modes for RAG query retrieval: - VECTOR: Uses vector similarity search
+        for semantic matching - KEYWORD: Uses keyword-based search for exact matching
+        - HYBRID: Combines both vector and keyword search for better results
     RRFRanker:
       type: object
       properties: