fix(native): ensure event ID for fastfail crashes

[jpnurmi]

This PR ensures that fast-fail crash envelopes have valid event IDs, which is required for resolving the correct envelope path when launching the external crash reporter.

For normal SEH crashes, native_backend_except is called, and it populates the event with a valid event ID by calling sentry_value_new_event. For fast-fail crashes captured via WER, however, the app callback cannot be called, and __sentry-event was therefore left without a valid ID. Consequently, the envelope's filename couldn't be correctly resolved when launching the external crash reporter for fast-fail crashes.

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