fix(packages): show error for duplicate cleanup rules #37820 (#38786)

## 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

routers/web/shared/packages/packages.go

@@ -117,6 +117,7 @@ func performRuleEditPost(ctx *context.Context, owner *user_model.User, pcr *pack
 			return
 		} else if has {
 			ctx.Data["Err_Type"] = true
+			ctx.Flash.Error(ctx.Tr("packages.owner.settings.cleanuprules.type.already_exists"), true)
 			ctx.HTML(http.StatusOK, template)
 			return
 		}