chore: add storage sane defaults (#4182)

# What does this PR do?

since `StackRunConfig` requires certain parts of `StorageConfig`, it'd
probably make sense to template in some defaults that will "just work"
for most usecases

specifically introduce`ServerStoresConfig` defaults for inference,
metadata, conversations and prompts. We already actually funnel in
defaults for these sections ad-hoc throughout the codebase

additionally set some `backends` defaults for the `StorageConfig`.

This will alleviate some weirdness for `--providers` for run/list-deps
and also some work I have to better align our list-deps/run datatypes

---------

Signed-off-by: Charlie Doern <cdoern@redhat.com>

src/llama_stack/core/storage/datatypes.py

@@ -12,6 +12,8 @@ from typing import Annotated, Literal
 
 from pydantic import BaseModel, Field, field_validator
 
+from llama_stack.core.utils.config_dirs import DISTRIBS_BASE_DIR
+
 
 class StorageBackendType(StrEnum):
     KV_REDIS = "kv_redis"
@@ -256,15 +258,24 @@ class ResponsesStoreReference(InferenceStoreReference):
 
 class ServerStoresConfig(BaseModel):
     metadata: KVStoreReference | None = Field(
-        default=None,
+        default=KVStoreReference(
+            backend="kv_default",
+            namespace="registry",
+        ),
         description="Metadata store configuration (uses KV backend)",
     )
     inference: InferenceStoreReference | None = Field(
-        default=None,
+        default=InferenceStoreReference(
+            backend="sql_default",
+            table_name="inference_store",
+        ),
         description="Inference store configuration (uses SQL backend)",
     )
     conversations: SqlStoreReference | None = Field(
-        default=None,
+        default=SqlStoreReference(
+            backend="sql_default",
+            table_name="openai_conversations",
+        ),
         description="Conversations store configuration (uses SQL backend)",
     )
     responses: ResponsesStoreReference | None = Field(
@@ -272,13 +283,21 @@ class ServerStoresConfig(BaseModel):
         description="Responses store configuration (uses SQL backend)",
     )
     prompts: KVStoreReference | None = Field(
-        default=None,
+        default=KVStoreReference(backend="kv_default", namespace="prompts"),
         description="Prompts store configuration (uses KV backend)",
     )
 
 
 class StorageConfig(BaseModel):
     backends: dict[str, StorageBackendConfig] = Field(
+        default={
+            "kv_default": SqliteKVStoreConfig(
+                db_path=f"${{env.SQLITE_STORE_DIR:={DISTRIBS_BASE_DIR}}}/kvstore.db",
+            ),
+            "sql_default": SqliteSqlStoreConfig(
+                db_path=f"${{env.SQLITE_STORE_DIR:={DISTRIBS_BASE_DIR}}}/sql_store.db",
+            ),
+        },
         description="Named backend configurations (e.g., 'default', 'cache')",
     )
     stores: ServerStoresConfig = Field(