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)

tests/unit/rag/test_rag_query.py

@@ -8,6 +8,7 @@ from unittest.mock import AsyncMock, MagicMock
 
 import pytest
 
+from llama_stack.apis.tools.rag_tool import RAGQueryConfig
 from llama_stack.apis.vector_io import (
     Chunk,
     ChunkMetadata,
@@ -58,3 +59,14 @@ class TestRagQuery:
         )
         assert expected_metadata_string in result.content[1].text
         assert result.content is not None
+
+    async def test_query_raises_incorrect_mode(self):
+        with pytest.raises(ValueError):
+            RAGQueryConfig(mode="invalid_mode")
+
+    @pytest.mark.asyncio
+    async def test_query_accepts_valid_modes(self):
+        RAGQueryConfig()  # Test default (vector)
+        RAGQueryConfig(mode="vector")  # Test vector
+        RAGQueryConfig(mode="keyword")  # Test keyword
+        RAGQueryConfig(mode="hybrid")  # Test hybrid