Fix load_directory silently dropping all descriptors

[zrgt]

Fixes #544

Problem

load_directory() called read_server_aas_json_file_into(), which internally only adds items where isinstance(item, model.Identifiable). AssetAdministrationShellDescriptor and SubmodelDescriptor are not Identifiable, so all descriptors were silently skipped. The registry always started empty.

Fix

Parse descriptor JSON directly with ServerAASFromJsonDecoder and add items to DictDescriptorStore via an isinstance(item, Descriptor) check.