Fix discriminator ambiguity with context-aware backend parsing

- Both SqliteKVStoreConfig and SqliteSqlStoreConfig use type='sqlite'
- Pydantic cannot distinguish them in a union
- Solution: Custom validator parses backends based on which stores reference them
- Metadata store requires KVStore, inference/conversations require SqlStore
- Separate kvstore/sqlstore backends in configs for clarity

llama_stack/distributions/starter/run.yaml

@@ -220,14 +220,17 @@ providers:
         db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/starter}/batches.db
 persistence:
   backends:
-    default:
+    kvstore:
       type: sqlite
-      db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/starter}/store.db
+      db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/starter}/kvstore.db
+    sqlstore:
+      type: sqlite
+      db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/starter}/sqlstore.db
   stores:
     metadata:
-      backend: default
+      backend: kvstore
     inference:
-      backend: default
+      backend: sqlstore
 models: []
 shields:
 - shield_id: llama-guard