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/core/test_storage_references.py

@@ -0,0 +1,84 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+"""Unit tests for storage backend/reference validation."""
+
+import pytest
+from pydantic import ValidationError
+
+from llama_stack.core.datatypes import (
+    LLAMA_STACK_RUN_CONFIG_VERSION,
+    StackRunConfig,
+)
+from llama_stack.core.storage.datatypes import (
+    InferenceStoreReference,
+    KVStoreReference,
+    ServerStoresConfig,
+    SqliteKVStoreConfig,
+    SqliteSqlStoreConfig,
+    SqlStoreReference,
+    StorageConfig,
+)
+
+
+def _base_run_config(**overrides):
+    metadata_reference = overrides.pop(
+        "metadata_reference",
+        KVStoreReference(backend="kv_default", namespace="registry"),
+    )
+    inference_reference = overrides.pop(
+        "inference_reference",
+        InferenceStoreReference(backend="sql_default", table_name="inference"),
+    )
+    conversations_reference = overrides.pop(
+        "conversations_reference",
+        SqlStoreReference(backend="sql_default", table_name="conversations"),
+    )
+    storage = overrides.pop(
+        "storage",
+        StorageConfig(
+            backends={
+                "kv_default": SqliteKVStoreConfig(db_path="/tmp/kv.db"),
+                "sql_default": SqliteSqlStoreConfig(db_path="/tmp/sql.db"),
+            },
+            stores=ServerStoresConfig(
+                metadata=metadata_reference,
+                inference=inference_reference,
+                conversations=conversations_reference,
+            ),
+        ),
+    )
+    return StackRunConfig(
+        version=LLAMA_STACK_RUN_CONFIG_VERSION,
+        image_name="test-distro",
+        apis=[],
+        providers={},
+        storage=storage,
+        **overrides,
+    )
+
+
+def test_references_require_known_backend():
+    with pytest.raises(ValidationError, match="unknown backend 'missing'"):
+        _base_run_config(metadata_reference=KVStoreReference(backend="missing", namespace="registry"))
+
+
+def test_references_must_match_backend_family():
+    with pytest.raises(ValidationError, match="kv_.* is required"):
+        _base_run_config(metadata_reference=KVStoreReference(backend="sql_default", namespace="registry"))
+
+    with pytest.raises(ValidationError, match="sql_.* is required"):
+        _base_run_config(
+            inference_reference=InferenceStoreReference(backend="kv_default", table_name="inference"),
+        )
+
+
+def test_valid_configuration_passes_validation():
+    config = _base_run_config()
+    stores = config.storage.stores
+    assert stores.metadata is not None and stores.metadata.backend == "kv_default"
+    assert stores.inference is not None and stores.inference.backend == "sql_default"
+    assert stores.conversations is not None and stores.conversations.backend == "sql_default"