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/conversations/test_conversations.py

@@ -20,7 +20,14 @@ from llama_stack.core.conversations.conversations import (
     ConversationServiceConfig,
     ConversationServiceImpl,
 )
-from llama_stack.providers.utils.sqlstore.sqlstore import SqliteSqlStoreConfig
+from llama_stack.core.datatypes import StackRunConfig
+from llama_stack.core.storage.datatypes import (
+    ServerStoresConfig,
+    SqliteSqlStoreConfig,
+    SqlStoreReference,
+    StorageConfig,
+)
+from llama_stack.providers.utils.sqlstore.sqlstore import register_sqlstore_backends
 
 
 @pytest.fixture
@@ -28,7 +35,18 @@ async def service():
     with tempfile.TemporaryDirectory() as tmpdir:
         db_path = Path(tmpdir) / "test_conversations.db"
 
-        config = ConversationServiceConfig(conversations_store=SqliteSqlStoreConfig(db_path=str(db_path)), policy=[])
+        storage = StorageConfig(
+            backends={
+                "sql_test": SqliteSqlStoreConfig(db_path=str(db_path)),
+            },
+            stores=ServerStoresConfig(
+                conversations=SqlStoreReference(backend="sql_test", table_name="openai_conversations"),
+            ),
+        )
+        register_sqlstore_backends({"sql_test": storage.backends["sql_test"]})
+        run_config = StackRunConfig(image_name="test", apis=[], providers={}, storage=storage)
+
+        config = ConversationServiceConfig(run_config=run_config, policy=[])
         service = ConversationServiceImpl(config, {})
         await service.initialize()
         yield service
@@ -121,9 +139,18 @@ async def test_policy_configuration():
             AccessRule(forbid=Scope(principal="test_user", actions=[Action.CREATE, Action.READ], resource="*"))
         ]
 
-        config = ConversationServiceConfig(
-            conversations_store=SqliteSqlStoreConfig(db_path=str(db_path)), policy=restrictive_policy
+        storage = StorageConfig(
+            backends={
+                "sql_test": SqliteSqlStoreConfig(db_path=str(db_path)),
+            },
+            stores=ServerStoresConfig(
+                conversations=SqlStoreReference(backend="sql_test", table_name="openai_conversations"),
+            ),
         )
+        register_sqlstore_backends({"sql_test": storage.backends["sql_test"]})
+        run_config = StackRunConfig(image_name="test", apis=[], providers={}, storage=storage)
+
+        config = ConversationServiceConfig(run_config=run_config, policy=restrictive_policy)
         service = ConversationServiceImpl(config, {})
         await service.initialize()