test: add wallet keychains smoke coverage

[j-kon]

Summary

• add deterministic offline smoke coverage for Wallet.keychains()
• assert returned keychains can be used from Dart to fetch public descriptors
• assert each returned WalletKeychain.publicDescriptor converts to a descriptor string

Closes #104

Validation

• dart format --output=none --set-exit-if-changed lib test example
• dart analyze --fatal-infos --fatal-warnings lib test example
• dart test