Fix assetIds decoding: return 400 instead of 500 on malformed JSON

[zrgt]

Fixes #502

Changes

• Wrap asset_dict["name"] / asset_dict["value"] in a try/except KeyError and raise BadRequest with a descriptive message — client input validation errors must return 4xx, not 5xx
• Add test_malformed_asset_id_missing_field_returns_400 unit test using Werkzeug test client