Enable nilnil linter for new code (#36591)

Fixes: https://github.com/go-gitea/gitea/issues/36152

Enable the `nilnil` linter while adding `//nolint` comments to existing
violations. This will ensure no new issues enter the code base while we
can fix existing issues gradually.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

services/contexttest/context_tests.go

@@ -192,7 +192,7 @@ func LoadGitRepo(t *testing.T, ctx gocontext.Context) {
 type MockRender struct{}
 
 func (tr *MockRender) TemplateLookup(tmpl string, _ gocontext.Context) (templates.TemplateExecutor, error) {
-	return nil, nil
+	return nil, nil //nolint:nilnil // mock implementation returns nil to indicate no template found
 }
 
 func (tr *MockRender) HTML(w io.Writer, status int, _ templates.TplName, _ any, _ gocontext.Context) error {