fix(identity-wallet): update tests for serde_json::Value and ChangeType enum
- Updated test data to use serde_json::json\!() for signed_op values
- Changed verified_signed_op test data from String to serde_json::Value
- Fixed OpDiff prev_cid to use Option<String> (Some/None) in tests
- Updated ServiceChange test to use ChangeType::Modified enum instead of string