Fix MultipleObjectsReturned error when multiple integrations exist (#12581)
Fixes a error that occurs when a project has multiple integrations with
the same .
## Changes
Modified method in to:
- Check the count of matching integrations
- Raise a clear (400 Bad Request) when multiple integrations are found
- Provide an actionable error message directing users to use the webhook
URL specific to their integration
## References
* Sentry issue: https://read-the-docs.sentry.io/issues/4951512277/
* Related issue:
https://github.com/readthedocs/readthedocs.org/issues/11037
---
*Generated by Copilot*
---------
Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>