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/server/test_resolver.py

@@ -12,15 +12,22 @@ from unittest.mock import AsyncMock, MagicMock
 from pydantic import BaseModel, Field
 
 from llama_stack.apis.inference import Inference
-from llama_stack.core.datatypes import (
-    Api,
-    Provider,
-    StackRunConfig,
-)
+from llama_stack.core.datatypes import Api, Provider, StackRunConfig
 from llama_stack.core.resolver import resolve_impls
 from llama_stack.core.routers.inference import InferenceRouter
 from llama_stack.core.routing_tables.models import ModelsRoutingTable
+from llama_stack.core.storage.datatypes import (
+    InferenceStoreReference,
+    KVStoreReference,
+    ServerStoresConfig,
+    SqliteKVStoreConfig,
+    SqliteSqlStoreConfig,
+    SqlStoreReference,
+    StorageConfig,
+)
 from llama_stack.providers.datatypes import InlineProviderSpec, ProviderSpec
+from llama_stack.providers.utils.kvstore import register_kvstore_backends
+from llama_stack.providers.utils.sqlstore.sqlstore import register_sqlstore_backends
 
 
 def add_protocol_methods(cls: type, protocol: type[Protocol]) -> None:
@@ -65,6 +72,35 @@ class SampleImpl:
         pass
 
 
+def make_run_config(**overrides) -> StackRunConfig:
+    storage = overrides.pop(
+        "storage",
+        StorageConfig(
+            backends={
+                "kv_default": SqliteKVStoreConfig(db_path=":memory:"),
+                "sql_default": SqliteSqlStoreConfig(db_path=":memory:"),
+            },
+            stores=ServerStoresConfig(
+                metadata=KVStoreReference(backend="kv_default", namespace="registry"),
+                inference=InferenceStoreReference(backend="sql_default", table_name="inference_store"),
+                conversations=SqlStoreReference(backend="sql_default", table_name="conversations"),
+            ),
+        ),
+    )
+    register_kvstore_backends({name: cfg for name, cfg in storage.backends.items() if cfg.type.value.startswith("kv_")})
+    register_sqlstore_backends(
+        {name: cfg for name, cfg in storage.backends.items() if cfg.type.value.startswith("sql_")}
+    )
+    defaults = dict(
+        image_name="test_image",
+        apis=[],
+        providers={},
+        storage=storage,
+    )
+    defaults.update(overrides)
+    return StackRunConfig(**defaults)
+
+
 async def test_resolve_impls_basic():
     # Create a real provider spec
     provider_spec = InlineProviderSpec(
@@ -78,7 +114,7 @@ async def test_resolve_impls_basic():
     # Create provider registry with our provider
     provider_registry = {Api.inference: {provider_spec.provider_type: provider_spec}}
 
-    run_config = StackRunConfig(
+    run_config = make_run_config(
         image_name="test_image",
         providers={
             "inference": [