fix(sentry-apps): gate error webhook subscriptions correctly

[cvxluo]

I slightly misconfigured the flag checks when GAing granular webhook subscriptions, so that in theory someone could pass error.created directly to the API and bypass the appropriate flag check for error events. Clean up that path.

[sentry-warden]

has_error_events iterates over raw request events without type guard

has_error_events is called on unvalidated request.data['events'] before the serializer runs. A non-iterable value such as an integer raises TypeError instead of a validation error.

Evidence

• has_error_events is called from sentry_apps.py:post() and sentry_app_details.py:put() on request.data.get('events') before SentryAppParser serializes the payload.
• The generator for event in events or () assumes events is iterable; a JSON body like {"events": 42} yields 42, which is not iterable and raises TypeError.
• The parser's EventListField would also reject an int, but has_error_events runs first and turns a validation error into an unhandled 500.

_{Identified by Warden · sentry-backend-bugs · 4GZ-TU4}