[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/providers/tests/agents/conftest.py

@@ -12,10 +12,10 @@ from ..conftest import (
     get_test_config_for_api,
 )
 from ..inference.fixtures import INFERENCE_FIXTURES
-from ..memory.fixtures import MEMORY_FIXTURES
 from ..safety.fixtures import SAFETY_FIXTURES, safety_model_from_shield
 
 from ..tools.fixtures import TOOL_RUNTIME_FIXTURES
+from ..vector_io.fixtures import VECTOR_IO_FIXTURES
 from .fixtures import AGENTS_FIXTURES
 
 DEFAULT_PROVIDER_COMBINATIONS = [
@@ -23,7 +23,7 @@ DEFAULT_PROVIDER_COMBINATIONS = [
         {
             "inference": "meta_reference",
             "safety": "llama_guard",
-            "memory": "faiss",
+            "vector_io": "faiss",
             "agents": "meta_reference",
             "tool_runtime": "memory_and_search",
         },
@@ -34,7 +34,7 @@ DEFAULT_PROVIDER_COMBINATIONS = [
         {
             "inference": "ollama",
             "safety": "llama_guard",
-            "memory": "faiss",
+            "vector_io": "faiss",
             "agents": "meta_reference",
             "tool_runtime": "memory_and_search",
         },
@@ -46,7 +46,7 @@ DEFAULT_PROVIDER_COMBINATIONS = [
             "inference": "together",
             "safety": "llama_guard",
             # make this work with Weaviate which is what the together distro supports
-            "memory": "faiss",
+            "vector_io": "faiss",
             "agents": "meta_reference",
             "tool_runtime": "memory_and_search",
         },
@@ -57,7 +57,7 @@ DEFAULT_PROVIDER_COMBINATIONS = [
         {
             "inference": "fireworks",
             "safety": "llama_guard",
-            "memory": "faiss",
+            "vector_io": "faiss",
             "agents": "meta_reference",
             "tool_runtime": "memory_and_search",
         },
@@ -68,7 +68,7 @@ DEFAULT_PROVIDER_COMBINATIONS = [
         {
             "inference": "remote",
             "safety": "remote",
-            "memory": "remote",
+            "vector_io": "remote",
             "agents": "remote",
             "tool_runtime": "memory_and_search",
         },
@@ -115,7 +115,7 @@ def pytest_generate_tests(metafunc):
         available_fixtures = {
             "inference": INFERENCE_FIXTURES,
             "safety": SAFETY_FIXTURES,
-            "memory": MEMORY_FIXTURES,
+            "vector_io": VECTOR_IO_FIXTURES,
             "agents": AGENTS_FIXTURES,
             "tool_runtime": TOOL_RUNTIME_FIXTURES,
         }