test: smoke test Dart full-scan binding call path

### Description
Add a narrow Dart-side smoke test for the generated full-scan binding path.

This is not intended to test Electrum/Esplora scan semantics or wallet discovery behavior. Those belong in `bdk-ffi`. The goal here is only to prove that the Dart-generated full-scan request/update path is callable and that the returned FFI objects lift into usable Dart objects.

Current Dart integration tests exercise `startSyncWithRevealedSpks()` and backend `sync_()`, but they do not call the generated full-scan path from Dart.

### Scope
- Build a wallet in Dart.
- Call `wallet.startFullScan().build()`.
- Pass the resulting `FullScanRequest` through one env-gated backend full-scan call.
- Assert the returned `Update` object lifts successfully and can be passed to `wallet.applyUpdate(...)`.
- Keep assertions limited to Dart binding callability and object usability.

Start with the existing Dart integration tests covering startSyncWithRevealedSpks() and backend sync_(), then trace the generated wallet.startFullScan().build() path. Add an environment-gated backend full-scan call, verify the returned Update lifts into a usable Dart object, and pass it to wallet.applyUpdate(...).

Written by the indexing model from the issue text.