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

tests/integration/api_packages_test.go

@@ -29,6 +29,33 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+func TestPackageCleanupRuleDuplicateType(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	user := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 4})
+	session := loginUser(t, user.Name)
+
+	// Create the first cleanup rule for the generic package type.
+	req := NewRequestWithValues(t, "POST", "/user/settings/packages/rules/add", map[string]string{
+		"type":        "generic",
+		"action":      "save",
+		"keep_count":  "0",
+		"remove_days": "0",
+	})
+	session.MakeRequest(t, req, http.StatusSeeOther)
+
+	// Try to create another cleanup rule for the same package type.
+	req = NewRequestWithValues(t, "POST", "/user/settings/packages/rules/add", map[string]string{
+		"type":        "generic",
+		"action":      "save",
+		"keep_count":  "0",
+		"remove_days": "0",
+	})
+
+	resp := session.MakeRequest(t, req, http.StatusOK)
+	assert.Contains(t, resp.Body.String(), "A cleanup rule for this package type already exists.")
+}
+
 func TestPackageAPI(t *testing.T) {
 	defer tests.PrepareTestEnv(t)()