fix: search mode validation for rag query (#2857)

# What does this PR do?
<!-- Provide a short summary of what this PR does and why. Link to
relevant issues if applicable. -->
I noticed a few issues with my implementation of the search mode
validation for RagQuery.
This PR replaces the check for search mode in RagQuery with a Literal. 
There were issues before with
```
TypeError: Object of type RAGSearchMode is not JSON serializable
```
When using 
```
query_config = RAGQueryConfig(max_chunks=6, mode="vector").model_dump()
```

It also fixes the fact that despite user input "vector" was always the
used search mode.
<!-- If resolving an issue, uncomment and update the line below -->
<!-- Closes #[issue-number] -->

## 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.* -->

Verify that a chosen search mode works when using Rag Query or use below
agent 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": "keyword",
                    "max_chunks": 6
                }
            },
        }
    ],
)
```

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

llama_stack/apis/tools/rag_tool.py

@@ -4,7 +4,7 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
-from enum import Enum
+from enum import Enum, StrEnum
 from typing import Annotated, Any, Literal, Protocol
 
 from pydantic import BaseModel, Field, field_validator
@@ -88,7 +88,7 @@ class RAGQueryGenerator(Enum):
 
 
 @json_schema_type
-class RAGSearchMode(Enum):
+class RAGSearchMode(StrEnum):
     """
     Search modes for RAG query retrieval:
     - VECTOR: Uses vector similarity search for semantic matching

tests/unit/rag/test_rag_query.py

@@ -65,7 +65,15 @@ class TestRagQuery:
             RAGQueryConfig(mode="invalid_mode")
 
     async def test_query_accepts_valid_modes(self):
-        RAGQueryConfig()  # Test default (vector)
+        default_config = RAGQueryConfig()  # Test default (vector)
-        RAGQueryConfig(mode="vector")  # Test vector
+        assert default_config.mode == "vector"
-        RAGQueryConfig(mode="keyword")  # Test keyword
+        vector_config = RAGQueryConfig(mode="vector")  # Test vector
-        RAGQueryConfig(mode="hybrid")  # Test hybrid
+        assert vector_config.mode == "vector"
+        keyword_config = RAGQueryConfig(mode="keyword")  # Test keyword
+        assert keyword_config.mode == "keyword"
+        hybrid_config = RAGQueryConfig(mode="hybrid")  # Test hybrid
+        assert hybrid_config.mode == "hybrid"
+
+        # Test that invalid mode raises an error
+        with pytest.raises(ValueError):
+            RAGQueryConfig(mode="wrong_mode")