test: smoke test repeated Wallet.keychains() calls across Dart wallet lifecycle

Description

Add a narrow Dart-side smoke test that calls Wallet.keychains() more than once across a simple wallet lifecycle.

This is not intended to expand the randomized sequence canary or retest wallet keychain behavior owned by bdk-ffi. The goal is only to prove that the generated Dart binding path remains callable after Dart side wallet operations such as persist and reload.

This complements #104: #104 verifies the returned List<WalletKeychain> lifts into usable Dart objects; this issue verifies the same generated path can be called again after the wallet has gone through a basic lifecycle.

Scope

• Create a wallet in Dart.
• Call wallet.keychains() once before persistence.
• Persist and reload the wallet using sqlite.
• Call wallet.keychains() again after reload.
• Keep assertions limited to binding callability and returned Dart object usability.