test: add request inspector callback smoke coverage

[ANAMASGARD]

Closes #107

Summary

• add a Dart-side smoke test for the generated SyncScriptInspector callback/vtable path
• run a minimal local Esplora-compatible server in a separate isolate
• verify that the inspector callback is invoked during request processing
• verify that the lifted Script and total arguments are usable Dart values

Why a separate isolate?

EsploraClient.sync_() blocks the calling Dart isolate through FFI. The fake HTTP server therefore runs in a separate isolate so it can continue accepting requests during the synchronous native call.

Scope

This test only covers generated callback callability and argument lifting. It does not test synchronization semantics, which remain the responsibility of bdk-ffi.

Verification

• dart test test/request_inspector_callback_test.dart -r expanded
• dart format --output=none --set-exit-if-changed lib test example
• dart analyze --fatal-infos --fatal-warnings lib test example
• dart test

--- ---