LCORE-1426: add SUPPORTED_BACKENDS mapping and restore doc comments

- Add SUPPORTED_BACKENDS dict to validate backend types during
  enrichment instead of hardcoding inline:: prefix
- Restore inline comments in reranker auto-enable validator
- Add backward compatibility note to RAG guide

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: are-ces

docs/rag_guide.md

@@ -34,6 +34,11 @@ Lightspeed Core Stack (LCS) supports two complementary RAG strategies:
 
 Both strategies can be enabled independently via the `rag` section of `lightspeed-stack.yaml`. See [BYOK Feature Documentation](byok_guide.md) for configuration details.
 
+> [!NOTE]
+> **Backward compatibility:** if neither `retrieval.inline.sources` nor `retrieval.tool.sources` is
+> configured, all BYOK vector stores registered in llama-stack are automatically exposed as
+> Tool RAG (`file_search`). Inline RAG is **not** enabled in this fallback — only Tool RAG.
+
 ### Inline RAG chunk flow
 
 ```mermaid

src/llama_stack_configuration.py

@@ -17,6 +17,11 @@
 
 logger = get_logger(__name__)
 
+BACKEND_TO_LLAMA_STACK_PROVIDER: dict[str, str] = {
+    "faiss": "inline::faiss",
+    # "pgvector": "remote::pgvector",  # TODO(are-ces): add enrichment support
+}
+
 
 class YamlDumper(yaml.Dumper):  # pylint: disable=too-many-ancestors
     """Custom YAML dumper with proper indentation levels."""
@@ -335,13 +340,22 @@ def construct_vector_io_providers_section(
             continue
         existing_ids.add(provider_id)
         added += 1
+
+        backend = brag.get("backend", constants.DEFAULT_RAG_BACKEND)
+        provider_type = BACKEND_TO_LLAMA_STACK_PROVIDER.get(backend)
+        if provider_type is None:
+            raise ValueError(
+                f"Unsupported backend '{backend}' for BYOK RAG '{rag_id}'. "
+                f"Supported backends: {list(BACKEND_TO_LLAMA_STACK_PROVIDER.keys())}"
+            )
+
         output.append(
             {
                 "provider_id": provider_id,
-                "provider_type": f"inline::{brag.get('backend', 'faiss')}",
+                "provider_type": provider_type,
                 "config": {
                     "persistence": {
-                        "namespace": "vector_io::faiss",
+                        "namespace": f"vector_io::{backend}",
                         "backend": backend_name,
                     }
                 },

src/models/config.py

@@ -2394,10 +2394,15 @@ def validate_reranker_auto_enable(self) -> Self:
         Returns:
             Self: The validated configuration instance with reranker potentially enabled.
         """
+        # Check if BYOK RAG entries are configured
         # pylint: disable=no-member
         has_byok = len(self.rag.byok.stores) > 0
+
+        # Check if OKP is configured in inline RAG strategy
         has_okp = constants.OKP_RAG_ID in self.rag.retrieval.inline.sources
 
+        # If both BYOK and OKP are present and reranker is using default settings,
+        # ensure it's enabled for optimal results
         if (
             has_byok
             and has_okp