feat: add input validation for search mode of rag query config (#2275)

# What does this PR do?
Adds input validation for mode in RagQueryConfig
This will prevent users from inputting search modes other than `vector`
and `keyword` for the time being with `hybrid` to follow when that
functionality is implemented.

## Test Plan
[Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.*]
```
# Check out this PR and enter the LS directory
uv sync --extra dev
```
Run the quickstart
[example](https://llama-stack.readthedocs.io/en/latest/getting_started/#step-3-run-the-demo)
Alter the Agent to include a query_config
```
agent = Agent(
    client,
    model=model_id,
    instructions="You are a helpful assistant",
    tools=[
        {
            "name": "builtin::rag/knowledge_search",
            "args": {
                "vector_db_ids": [vector_db_id],
                "query_config": {
                    "mode": "i-am-not-vector", # Test for non valid search mode
                    "max_chunks": 6
                }
            },
        }
    ],
)
```
Ensure you get the following error:
```
400: {'errors': [{'loc': ['mode'], 'msg': "Value error, mode must be either 'vector' or 'keyword' if supported by the vector_io provider", 'type': 'value_error'}]}
```

## Running unit tests
```
uv sync --extra dev
uv run pytest tests/unit/rag/test_rag_query.py -v
```

[//]: # (## Documentation)

docs/_static/llama-stack-spec.yaml

@@ -10346,7 +10346,8 @@ 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".
@@ -10373,6 +10374,17 @@ 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: