[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/distribution/server/endpoints.py

@@ -9,6 +9,8 @@ from typing import Dict, List
 
 from pydantic import BaseModel
 
+from llama_stack.apis.tools import RAGToolRuntime, SpecialToolGroup
+
 from llama_stack.apis.version import LLAMA_STACK_API_VERSION
 
 from llama_stack.distribution.resolver import api_protocol_map
@@ -22,21 +24,39 @@ class ApiEndpoint(BaseModel):
     name: str
 
 
+def toolgroup_protocol_map():
+    return {
+        SpecialToolGroup.rag_tool: RAGToolRuntime,
+    }
+
+
 def get_all_api_endpoints() -> Dict[Api, List[ApiEndpoint]]:
     apis = {}
 
     protocols = api_protocol_map()
+    toolgroup_protocols = toolgroup_protocol_map()
     for api, protocol in protocols.items():
         endpoints = []
         protocol_methods = inspect.getmembers(protocol, predicate=inspect.isfunction)
 
+        # HACK ALERT
+        if api == Api.tool_runtime:
+            for tool_group in SpecialToolGroup:
+                sub_protocol = toolgroup_protocols[tool_group]
+                sub_protocol_methods = inspect.getmembers(
+                    sub_protocol, predicate=inspect.isfunction
+                )
+                for name, method in sub_protocol_methods:
+                    if not hasattr(method, "__webmethod__"):
+                        continue
+                    protocol_methods.append((f"{tool_group.value}.{name}", method))
+
         for name, method in protocol_methods:
             if not hasattr(method, "__webmethod__"):
                 continue
 
             webmethod = method.__webmethod__
             route = f"/{LLAMA_STACK_API_VERSION}/{webmethod.route.lstrip('/')}"
-
             if webmethod.method == "GET":
                 method = "get"
             elif webmethod.method == "DELETE":