### Summary
Fixed flaky Instagram webhook specs that failed intermittently in cloud
environments due to shared let blocks creating conflicting inboxes. The
Instagram channel factory already creates an inbox automatically, but
tests were adding extra ones in shared contexts.
Moved channel/inbox creation to isolated test contexts to prevent race
conditions between Facebook Page and Instagram Direct tests.
### Testing
```
for i in {1..30}; do
echo "=== Run $i ==="
RAILS_ENV=test bundle exec rspec spec/jobs/webhooks/instagram_events_job_spec.rb --fail-fast || break
done
```
Previously, intermittent failures could be reproduced locally. With
these changes, tests achieve ~100% pass rate.
ad4ec9e93b