Node-API: async-work complete callback runs without an entered context

[edusperoni]

Found while porting #437 to Android (NativeScript/android#2004) by an independent review of the port; the defect is shared with the iOS original.

CompleteAsyncWork in NativeScript/napi/NodeApiEmbed.mm calls env->CallIntoModule(...) with no v8::Context::Scope (and no HandleScope of its own) — the event-loop entry it runs in supplies only Locker + Isolate::Scope + HandleScope. Every sibling entry path opens the context scope itself (NapiEnv::CallFinalizer, the TSFN dispatch in NapiThreadSafeFunction.mm), but the async-work completion path does not.

Consequences:

• Node guarantees the complete callback runs with the env's context entered; addon code that touches the engine directly (or any napi call that reads isolate->GetCurrentContext()) observes an empty current context.
• If the complete callback throws JS-side (e.g. the documented napi_throw_error failure-reporting pattern), the exception handler runs against an empty current context — on the Android port this was a null-deref/CHECK crash in the error reporter; the iOS reporter may or may not survive it, but the contract violation is the same.

Fix is the same one-liner the Android port applied in review: open v8::HandleScope + v8::Context::Scope(env->context()) in CompleteAsyncWork before CallIntoModule, mirroring NapiEnv::CallFinalizer.

Android-side fix: NativeScript/android#2004 (CompleteAsyncWork in test-app/runtime/src/main/cpp/napi/NodeApiEmbed.cpp).