整理配置和代码入口，增加部分测试。

sync_state_machine/config/runtime_config.py

@@ -3,7 +3,7 @@
 Prefer importing from `sync_state_machine.config` directly.
 """
 
-from .datasource_config import ApiDataSourceConfig, JsonlDataSourceConfig, DataSourceConfig
+from .datasource_config import ApiDataSourceConfig, DbDataSourceConfig, JsonlDataSourceConfig, DataSourceConfig
 from .persist_config import PersistConfig
 from .logging_config import LoggingConfig
 from .strategy_config import (
@@ -24,6 +24,7 @@ from .builder import (
 
 __all__ = [
     "ApiDataSourceConfig",
+    "DbDataSourceConfig",
     "JsonlDataSourceConfig",
     "DataSourceConfig",
     "PersistConfig",