test: add Wallet.keychains() behavior coverage for standard and single-descriptor wallets

[Johnosezele]

Description

Add offline unit tests that call Wallet.keychains() and assert the returned List<WalletKeychain> is structurally correct for the two wallet shapes already covered elsewhere in the test suite.

#101 regenerated lib/bdk.dart with Wallet.keychains() and WalletKeychain, but existing tests never invoke this method.

Scope

• Cover a standard two-descriptor wallet built with buildBip84Descriptor / buildBip84ChangeDescriptor.
• Cover a single descriptor wallet built with Wallet.createSingle.