feat(stores)!: use backend storage references instead of configs (#3697)

**This PR changes configurations in a backward incompatible way.**

Run configs today repeat full SQLite/Postgres snippets everywhere a
store is needed, which means duplicated credentials, extra connection
pools, and lots of drift between files. This PR introduces named storage
backends so the stack and providers can share a single catalog and
reference those backends by name.

## Key Changes

- Add `storage.backends` to `StackRunConfig`, register each KV/SQL
backend once at startup, and validate that references point to the right
family.
- Move server stores under `storage.stores` with lightweight references
(backend + namespace/table) instead of full configs.
- Update every provider/config/doc to use the new reference style;
docs/codegen now surface the simplified YAML.

## Migration

Before:
```yaml
metadata_store:
  type: sqlite
  db_path: ~/.llama/distributions/foo/registry.db
inference_store:
  type: postgres
  host: ${env.POSTGRES_HOST}
  port: ${env.POSTGRES_PORT}
  db: ${env.POSTGRES_DB}
  user: ${env.POSTGRES_USER}
  password: ${env.POSTGRES_PASSWORD}
conversations_store:
  type: postgres
  host: ${env.POSTGRES_HOST}
  port: ${env.POSTGRES_PORT}
  db: ${env.POSTGRES_DB}
  user: ${env.POSTGRES_USER}
  password: ${env.POSTGRES_PASSWORD}
```

After:
```yaml
storage:
  backends:
    kv_default:
      type: kv_sqlite
      db_path: ~/.llama/distributions/foo/kvstore.db
    sql_default:
      type: sql_postgres
      host: ${env.POSTGRES_HOST}
      port: ${env.POSTGRES_PORT}
      db: ${env.POSTGRES_DB}
      user: ${env.POSTGRES_USER}
      password: ${env.POSTGRES_PASSWORD}
  stores:
    metadata:
      backend: kv_default
      namespace: registry
    inference:
      backend: sql_default
      table_name: inference_store
      max_write_queue_size: 10000
      num_writers: 4
    conversations:
      backend: sql_default
      table_name: openai_conversations
```

Provider configs follow the same pattern—for example, a Chroma vector
adapter switches from:

```yaml
providers:
  vector_io:
  - provider_id: chromadb
    provider_type: remote::chromadb
    config:
      url: ${env.CHROMADB_URL}
      kvstore:
        type: sqlite
        db_path: ~/.llama/distributions/foo/chroma.db
```

to:

```yaml
providers:
  vector_io:
  - provider_id: chromadb
    provider_type: remote::chromadb
    config:
      url: ${env.CHROMADB_URL}
      persistence:
        backend: kv_default
        namespace: vector_io::chroma_remote
```

Once the backends are declared, everything else just points at them, so
rotating credentials or swapping to Postgres happens in one place and
the stack reuses a single connection pool.

tests/unit/utils/responses/test_responses_store.py

@@ -6,6 +6,7 @@
 
 import time
 from tempfile import TemporaryDirectory
+from uuid import uuid4
 
 import pytest
 
@@ -15,8 +16,18 @@ from llama_stack.apis.agents.openai_responses import (
     OpenAIResponseObject,
 )
 from llama_stack.apis.inference import OpenAIMessageParam, OpenAIUserMessageParam
+from llama_stack.core.storage.datatypes import ResponsesStoreReference, SqliteSqlStoreConfig
 from llama_stack.providers.utils.responses.responses_store import ResponsesStore
-from llama_stack.providers.utils.sqlstore.sqlstore import SqliteSqlStoreConfig
+from llama_stack.providers.utils.sqlstore.sqlstore import register_sqlstore_backends
+
+
+def build_store(db_path: str, policy: list | None = None) -> ResponsesStore:
+    backend_name = f"sql_responses_{uuid4().hex}"
+    register_sqlstore_backends({backend_name: SqliteSqlStoreConfig(db_path=db_path)})
+    return ResponsesStore(
+        ResponsesStoreReference(backend=backend_name, table_name="responses"),
+        policy=policy or [],
+    )
 
 
 def create_test_response_object(
@@ -54,7 +65,7 @@ async def test_responses_store_pagination_basic():
     """Test basic pagination functionality for responses store."""
     with TemporaryDirectory() as tmp_dir:
         db_path = tmp_dir + "/test.db"
-        store = ResponsesStore(SqliteSqlStoreConfig(db_path=db_path), policy=[])
+        store = build_store(db_path)
         await store.initialize()
 
         # Create test data with different timestamps
@@ -103,7 +114,7 @@ async def test_responses_store_pagination_ascending():
     """Test pagination with ascending order."""
     with TemporaryDirectory() as tmp_dir:
         db_path = tmp_dir + "/test.db"
-        store = ResponsesStore(SqliteSqlStoreConfig(db_path=db_path), policy=[])
+        store = build_store(db_path)
         await store.initialize()
 
         # Create test data
@@ -141,7 +152,7 @@ async def test_responses_store_pagination_with_model_filter():
     """Test pagination combined with model filtering."""
     with TemporaryDirectory() as tmp_dir:
         db_path = tmp_dir + "/test.db"
-        store = ResponsesStore(SqliteSqlStoreConfig(db_path=db_path), policy=[])
+        store = build_store(db_path)
         await store.initialize()
 
         # Create test data with different models
@@ -182,7 +193,7 @@ async def test_responses_store_pagination_invalid_after():
     """Test error handling for invalid 'after' parameter."""
     with TemporaryDirectory() as tmp_dir:
         db_path = tmp_dir + "/test.db"
-        store = ResponsesStore(SqliteSqlStoreConfig(db_path=db_path), policy=[])
+        store = build_store(db_path)
         await store.initialize()
 
         # Try to paginate with non-existent ID
@@ -194,7 +205,7 @@ async def test_responses_store_pagination_no_limit():
     """Test pagination behavior when no limit is specified."""
     with TemporaryDirectory() as tmp_dir:
         db_path = tmp_dir + "/test.db"
-        store = ResponsesStore(SqliteSqlStoreConfig(db_path=db_path), policy=[])
+        store = build_store(db_path)
         await store.initialize()
 
         # Create test data
@@ -226,7 +237,7 @@ async def test_responses_store_get_response_object():
     """Test retrieving a single response object."""
     with TemporaryDirectory() as tmp_dir:
         db_path = tmp_dir + "/test.db"
-        store = ResponsesStore(SqliteSqlStoreConfig(db_path=db_path), policy=[])
+        store = build_store(db_path)
         await store.initialize()
 
         # Store a test response
@@ -254,7 +265,7 @@ async def test_responses_store_input_items_pagination():
     """Test pagination functionality for input items."""
     with TemporaryDirectory() as tmp_dir:
         db_path = tmp_dir + "/test.db"
-        store = ResponsesStore(SqliteSqlStoreConfig(db_path=db_path), policy=[])
+        store = build_store(db_path)
         await store.initialize()
 
         # Store a test response with many inputs with explicit IDs
@@ -335,7 +346,7 @@ async def test_responses_store_input_items_before_pagination():
     """Test before pagination functionality for input items."""
     with TemporaryDirectory() as tmp_dir:
         db_path = tmp_dir + "/test.db"
-        store = ResponsesStore(SqliteSqlStoreConfig(db_path=db_path), policy=[])
+        store = build_store(db_path)
         await store.initialize()
 
         # Store a test response with many inputs with explicit IDs