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

[Johnosezele]

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.

[ANAMASGARD]

@Johnosezele I have added the test and raised PR ran test locally also please review the PR and give feedback , Thanks