fix: add missing UnconfirmedTransaction import in error_handling_test.rs

Added the missing import for UnconfirmedTransaction type which is used in the
StorageManager trait implementation. This prevents compilation failure when
the cfg gate is eventually removed.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: 2 people

dash-spv/tests/error_handling_test.rs

@@ -36,7 +36,7 @@ use dash_spv::network::TcpConnection;
 use dash_spv::storage::{DiskStorageManager, MemoryStorageManager, StorageManager};
 use dash_spv::sync::sequential::phases::SyncPhase;
 use dash_spv::sync::sequential::recovery::{RecoveryManager, RecoveryStrategy};
-use dash_spv::types::{ChainState, MempoolState};
+use dash_spv::types::{ChainState, MempoolState, UnconfirmedTransaction};
 use dash_spv::wallet::Utxo;
 
 /// Mock network manager for testing error scenarios