test: improve test coverage

[darioAnongba]

Summary

Current tests are minimal (1 browser wasm-bindgen test, ~4 Node integration tests). Need comprehensive unit and integration tests.

Tasks

• Add unit tests for type conversions (From/Into impls)
• Add tests for error handling and edge cases
• Add tests for TxBuilder options (drain_wallet, ordering, fee_rate, exclude_unconfirmed, etc.)
• Add tests for BIP-389 multipath descriptor creation
• Add tests for SignOptions
• Add wallet lifecycle tests (create → sync → build tx → sign)
• Consider property-based testing for type conversions