Generates a new signing secret for the webhook endpoint. The previous secret is immediately invalidated.
The endpoint_id must be a prefixed ID (e.g. whe_9ZtRcXw3nByK5pVqMaEfJs).
^whe_[A-Za-z0-9]+$"whe_9ZtRcXw3nByK5pVqMaEfJs"
Signing secret rotated successfully.
The new signing secret. Store it securely — it cannot be retrieved again.