fix(errors): forward originalError for unhandledrejection via NaisErrorsInstrumentation

[sindrerh2]

What

Faro's built-in ErrorsInstrumentation forwards the actual Error instance as pushError's originalError option for window.onerror, but not for unhandledrejection (registerOnunhandledrejection only passes { type, stackFrames }). This means:

• With preserveOriginalError: true, a rejected promise's real Error never reached a custom beforeSend hook.
• Faro's built-in listener registers before an app's own unhandledrejection listener, so it pushes an originalError-less item first — a subsequent manual captureException(error) call for the same rejection gets silently dropped by Faro's dedupe.

Fix

Add NaisErrorsInstrumentation, a drop-in replacement for Faro's ErrorsInstrumentation:

• window.onerror behavior is unchanged (already forwarded originalError upstream).
• unhandledrejection uses the same logic as Faro's registerOnunhandledrejection (including its reason/detail fallback chain), with originalError now included.
• init() excludes the built-in ErrorsInstrumentation from getWebInstrumentations() and registers this one instead — exactly one listener per event type, no dedupe race.
• initialize() is idempotent; destroy() only restores a previously-installed window.onerror if nothing else replaced it since.

Adversarially reviewed (GPT-5.6-Sol) and 3 findings were fixed: an incorrect reason-fallback chain, a destroy() that could clobber a handler installed by other code later, and a missing idempotency guard.

Docs

README updated to describe this behavior and recommend removing redundant app-level unhandledrejection + captureException listeners now that originalError is included automatically.

Testing

• npx vitest run — 242 passed
• npx tsc --noEmit — clean
• npx tsc -p tsconfig.build.json — clean