test: add CreateParams / LoadParams wallet constructor smoke coverage

[Johnosezele]

Summary

• add offline smoke coverage for the generated CreateParams / LoadParams wallet constructors
• construct through Wallet.createWithParams and Wallet.createSingleWithParams
• persist staged derivation state, dispose the wallet and persister, and reopen through Wallet.loadWithParams and Wallet.loadSingleWithParams
• assert the reloaded wallets expose a usable network and can peek an address

Closes #135

Why

Existing tests cover Wallet(...), Wallet.createSingle, and Wallet.load. They do not call the *WithParams constructors added in bdk-ffi v3.1.0. This only proves those Dart binding paths are callable and that the returned wallets lift into usable objects.

[j-kon]

The new smoke tests look great!

Since both tests already call revealNextAddress(keychain: KeychainKind.external_) before persisting to SQLite, we could also verify that the persisted derivation state survived the reload by asserting the reloaded wallet's derivation index:

expect(
  reopenedWallet.derivationIndex(keychain: KeychainKind.external_),
  equals(0),
);

Tested locally against Wallet.loadWithParams and Wallet.loadSingleWithParams and it passes cleanly.