test: add Wallet.keychains() Dart binding smoke coverage

Description

Add a small Dart-side smoke test for the generated Wallet.keychains() binding.

This is not intended to retest wallet keychain behavior owned by bdk-ffi. The goal is only to prove the regenerated Dart binding path is callable and that List<WalletKeychain> lifts into usable Dart objects. Wallet.keychains() came in through #101, and the current Dart test suite does not call this generated path today.

Scope

• Call wallet.keychains() from Dart on a standard wallet.
• Assert the returned list is non-empty.
• Assert each returned WalletKeychain exposes a usable keychain.
• Assert each returned WalletKeychain.publicDescriptor can be converted to a descriptor string.
• Keep assertions thin and focused on Dart binding usability.