[memory refactor][3/n] Introduce RAGToolRuntime as a specialized sub-protocol (#832)

See https://github.com/meta-llama/llama-stack/issues/827 for the broader
design.

Third part:
- we need to make `tool_runtime.rag_tool.query_context()` and
`tool_runtime.rag_tool.insert_documents()` methods work smoothly with
complete type safety. To that end, we introduce a sub-resource path
`tool-runtime/rag-tool/` and make changes to the resolver to make things
work.
- the PR updates the agents implementation to directly call these typed
APIs for memory accesses rather than going through the complex, untyped
"invoke_tool" API. the code looks much nicer and simpler (expectedly.)
- there are a number of hacks in the server resolver implementation
still, we will live with some and fix some

Note that we must make sure the client SDKs are able to handle this
subresource complexity also. Stainless has support for subresources, so
this should be possible but beware.

## Test Plan

Our RAG test is sad (doesn't actually test for actual RAG output) but I
verified that the implementation works. I will work on fixing the RAG
test afterwards.

```bash
pytest -s -v tests/agents/test_agents.py -k "rag and together" --safety-shield=meta-llama/Llama-Guard-3-8B
```

llama_stack/apis/tools/tools.py

@@ -15,6 +15,8 @@ from llama_stack.apis.common.content_types import InterleavedContent, URL
 from llama_stack.apis.resource import Resource, ResourceType
 from llama_stack.providers.utils.telemetry.trace_protocol import trace_protocol
 
+from .rag_tool import RAGToolRuntime
+
 
 @json_schema_type
 class ToolParameter(BaseModel):
@@ -130,11 +132,17 @@ class ToolGroups(Protocol):
         ...
 
 
+class SpecialToolGroup(Enum):
+    rag_tool = "rag_tool"
+
+
 @runtime_checkable
 @trace_protocol
 class ToolRuntime(Protocol):
     tool_store: ToolStore
 
+    rag_tool: RAGToolRuntime
+
     # TODO: This needs to be renamed once OPEN API generator name conflict issue is fixed.
     @webmethod(route="/tool-runtime/list-tools", method="GET")
     async def list_runtime_tools(
@@ -143,7 +151,7 @@ class ToolRuntime(Protocol):
 
     @webmethod(route="/tool-runtime/invoke", method="POST")
     async def invoke_tool(
-        self, tool_name: str, args: Dict[str, Any]
+        self, tool_name: str, kwargs: Dict[str, Any]
     ) -> ToolInvocationResult:
         """Run a tool with the given arguments"""
         ...