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` 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.

Start by locating the existing Dart wallet lifecycle tests and the complementary coverage from #104. Exercise Wallet.keychains() before persistence and again after sqlite reload, keeping assertions to successful binding calls and usable returned Dart objects; done means both calls work across the lifecycle.

Written by the indexing model from the issue text.