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/prompts/prompts/conftest.py

@@ -9,7 +9,16 @@ import random
 import pytest
 
 from llama_stack.core.prompts.prompts import PromptServiceConfig, PromptServiceImpl
-from llama_stack.providers.utils.kvstore.config import SqliteKVStoreConfig
+from llama_stack.core.storage.datatypes import (
+    InferenceStoreReference,
+    KVStoreReference,
+    ServerStoresConfig,
+    SqliteKVStoreConfig,
+    SqliteSqlStoreConfig,
+    SqlStoreReference,
+    StorageConfig,
+)
+from llama_stack.providers.utils.kvstore import kvstore_impl, register_kvstore_backends
 
 
 @pytest.fixture
@@ -19,12 +28,28 @@ async def temp_prompt_store(tmp_path_factory):
     db_path = str(temp_dir / f"{unique_id}.db")
 
     from llama_stack.core.datatypes import StackRunConfig
-    from llama_stack.providers.utils.kvstore import kvstore_impl
 
-    mock_run_config = StackRunConfig(image_name="test-distribution", apis=[], providers={})
+    storage = StorageConfig(
+        backends={
+            "kv_test": SqliteKVStoreConfig(db_path=db_path),
+            "sql_test": SqliteSqlStoreConfig(db_path=str(temp_dir / f"{unique_id}_sql.db")),
+        },
+        stores=ServerStoresConfig(
+            metadata=KVStoreReference(backend="kv_test", namespace="registry"),
+            inference=InferenceStoreReference(backend="sql_test", table_name="inference"),
+            conversations=SqlStoreReference(backend="sql_test", table_name="conversations"),
+        ),
+    )
+    mock_run_config = StackRunConfig(
+        image_name="test-distribution",
+        apis=[],
+        providers={},
+        storage=storage,
+    )
     config = PromptServiceConfig(run_config=mock_run_config)
     store = PromptServiceImpl(config, deps={})
 
-    store.kvstore = await kvstore_impl(SqliteKVStoreConfig(db_path=db_path))
+    register_kvstore_backends({"kv_test": storage.backends["kv_test"]})
+    store.kvstore = await kvstore_impl(KVStoreReference(backend="kv_test", namespace="prompts"))
 
     yield store