test: add Dart callback/vtable smoke coverage for request inspectors

### Description
Add a narrow Dart-side smoke test for the generated callback/vtable path used by request inspectors.

This is not intended to test sync or full-scan semantics. Those behavior tests belong in `bdk-ffi`. The goal here is only to prove that the generated `uniffi-dart` callback wiring is callable from Dart and that callback arguments lift into usable Dart objects.

`SyncScriptInspector` and `FullScanScriptInspector` go through generated Dart callback/vtable code, so this path is worth a small binding-level smoke test in `bdk-dart`.

### Scope
- Add a minimal Dart inspector implementation that records callback invocations.
- Exercise one request-builder inspector path from Dart.
- Assert the callback is invoked.
- Assert callback arguments lift into usable Dart values.
- Keep assertions focused on callback callability and generated binding shape.

Start in bdk-dart by tracing the request-builder path through SyncScriptInspector and FullScanScriptInspector and the generated uniffi-dart callback/vtable bindings. Add a minimal Dart inspector, exercise one request-builder inspector path, and verify that the callback runs and its arguments are usable Dart values; sync and full-scan semantics remain covered in bdk-ffi.

Written by the indexing model from the issue text.