test(mail): assert the base partials stay compiled

The "base/" partials are excluded from MailRender.TemplateNames but must
remain in the compiled template set, otherwise every mail that includes
them fails to render. Guard the invariant with a test instead of a
comment.

Assisted-by: Claude:Opus 4.8

Author: silverwind

modules/templates/mail.go

@@ -130,7 +130,7 @@ func newMailRenderer() (*MailRender, error) {
 			renderer.TemplateNames = slices.DeleteFunc(slices.Clone(names), func(name string) bool {
 				return strings.HasPrefix(name, "base/")
 			})
-			return names, nil // the "base/" partials must stay compiled
+			return names, nil
 		},
 		readTemplateContent: func(name string) ([]byte, error) {
 			content, err := assetFS.ReadFile("mail/" + name + ".tmpl")

services/mailer/mail_templates_test.go

@@ -0,0 +1,20 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package mailer
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMailTemplatesBasePartials(t *testing.T) {
+	for _, name := range LoadedTemplates().TemplateNames {
+		assert.False(t, strings.HasPrefix(name, "base/"), "partial %q must not be listed as a mail", name)
+	}
+	for _, name := range []string{"base/head", "base/footer", "base/footer_copyright"} {
+		assert.True(t, LoadedTemplates().BodyTemplates.HasTemplate(name), "partial %q must stay compiled", name)
+	}
+}