## Issue
Gitea does not display a clear error message when a user tries to create
a cleanup rule for a package type that already has an existing cleanup
rule.
Although the duplicate rule is detected, the user is not informed why
the cleanup rule cannot be created.
## Solution
Add a user-facing error message when a cleanup rule already exists for
the selected package type.
Also add an integration test to verify that the appropriate error
message is displayed when attempting to create a duplicate cleanup rule.
Fixes#37820
