Fixes crash when platform/app destroys itself during the bootstrapping and
stabilization phase. Previously, the code would call `applicationRef.injector`
without checking if the platform was destroyed, resulting in:
"Error: Injector has already been destroyed"
This can occur when:
- Component constructor calls `inject(PlatformRef).destroy()`
- AbortSignal triggers during request handling
- APP_INITIALIZER rejects and causes cleanup
- Custom guard/resolver logic destroys the platform
Solution: Check `applicationRef.destroyed` after `whenStable()` and return error
state instead of accessing destroyed injector.
Test: Added test case that destroys app in component constructor to
verify graceful handling of this edge case.