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/providers/agent/test_meta_reference_agent.py

@@ -26,6 +26,20 @@ from llama_stack.providers.inline.agents.meta_reference.config import MetaRefere
 from llama_stack.providers.inline.agents.meta_reference.persistence import AgentInfo
 
 
+@pytest.fixture(autouse=True)
+def setup_backends(tmp_path):
+    """Register KV and SQL store backends for testing."""
+    from llama_stack.core.storage.datatypes import SqliteKVStoreConfig, SqliteSqlStoreConfig
+    from llama_stack.providers.utils.kvstore.kvstore import register_kvstore_backends
+    from llama_stack.providers.utils.sqlstore.sqlstore import register_sqlstore_backends
+
+    kv_path = str(tmp_path / "test_kv.db")
+    sql_path = str(tmp_path / "test_sql.db")
+
+    register_kvstore_backends({"kv_default": SqliteKVStoreConfig(db_path=kv_path)})
+    register_sqlstore_backends({"sql_default": SqliteSqlStoreConfig(db_path=sql_path)})
+
+
 @pytest.fixture
 def mock_apis():
     return {
@@ -40,15 +54,20 @@ def mock_apis():
 
 @pytest.fixture
 def config(tmp_path):
+    from llama_stack.core.storage.datatypes import KVStoreReference, ResponsesStoreReference
+    from llama_stack.providers.inline.agents.meta_reference.config import AgentPersistenceConfig
+
     return MetaReferenceAgentsImplConfig(
-        persistence_store={
-            "type": "sqlite",
-            "db_path": str(tmp_path / "test.db"),
-        },
-        responses_store={
-            "type": "sqlite",
-            "db_path": str(tmp_path / "test.db"),
-        },
+        persistence=AgentPersistenceConfig(
+            agent_state=KVStoreReference(
+                backend="kv_default",
+                namespace="agents",
+            ),
+            responses=ResponsesStoreReference(
+                backend="sql_default",
+                table_name="responses",
+            ),
+        )
     )