AppRuntime: route unhandled promise rejections to the host handler (V8) — hop 3 of #196

[bkaradzic-microsoft]

Hop 3 of #196 — make fire-and-forget promise rejections reach the embedding app instead of vanishing.

Problem

AppRuntime::Dispatch only catches synchronous Napi::Error throws from dispatched callbacks, and no engine had a promise-rejection tracker wired anywhere in Core. So a fire-and-forget fetch() that rejects (or a throw inside any .then with no .catch) never reached the embedder's UnhandledExceptionHandler — the Sentry/Bugsnag hook — and the process exited 0.

Approach

• AppRuntime::Options::EnableUnhandledPromiseRejectionTracking (opt-in, default false). When false, no tracker is installed and behavior is unchanged. When true, unhandled rejections route into the existing UnhandledExceptionHandler.
• AppRuntime::OnUnhandledPromiseRejection(napi_value) wraps the rejection reason as a Napi::Error (forwarding Error-like reasons as-is to preserve message/stack/cause; stringifying non-objects) and invokes the handler.
• Deferred, Node-like semantics: reporting is deferred to the end of the turn via Dispatch, so a rejection that is handled synchronously in the same turn (const p = Promise.reject(e); p.catch(...)) is dropped before it is ever reported — no false positives.

Engine support

Engine	Status
V8	✅ Implemented + tested (Isolate::SetPromiseRejectCallback; promise/reason held in v8::Global across the deferred flush).
Chakra	⚠️ Documented no-op — the OS EdgeMode runtime (chakrart.h) exposes no host promise-rejection tracker (JsSetHostPromiseRejectionTracker is ChakraCore-only, and JsRuntimeHost builds against EdgeMode via USE_EDGEMODE_JSRT).
JavaScriptCore / JSI	⏳ Follow-ups — JSGlobalContextSetUnhandledRejectionCallback (JSC) and the JSI tracker. Happy to add in this PR or separately; would value guidance on the JSI/V8JSI path.

Validation

Native tests in Shared.cpp (skipped on the Chakra backend, which can't support the feature):

• UnhandledPromiseRejectionReachesHandler — a fire-and-forget Promise.reject(new Error('boom')) reaches the host handler with the right message.
• SynchronouslyHandledRejectionDoesNotReachHandler — a synchronously-handled rejection does not reach the handler (verifies the deferral).

Built and ran on Win32 / V8 (both pass) and Win32 / Chakra (framework compiles, both tests skip as expected). The opt-in default keeps the existing JavaScript, All suite unaffected.

Notes for reviewers

• OnUnhandledPromiseRejection is exposed as a public AppRuntime method (documented "internal, engine-implementation use") because the V8 rejection callback is a bare function pointer whose helpers are free functions and can't reach a private member.
• The V8 backend reuses the internal reinterpret_cast<napi_value>(*local) bridge (mirrored from js_native_api_v8.h, which isn't on the public include path) with the same static_assert.

Part of the #196 chain (hop 2 is #200).

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[bkaradzic-microsoft]

Consolidated into #200 — the whole #196 chain is now a single PR (hop 3 is the second commit there). Closing to avoid three PRs for one feature in one repo. The unhandled-rejection work, all CI fixes (V8-gating, the JSI napi.h build fix, and the Android JNI engine define), and the native tests are preserved in #200.