[BUG] Don't remove builtin OAuth2 applications

- When the database consistency is being run it would check for any
OAuth2 applications that don't have an existing user. However there are
few special OAuth2 applications that don't have an user set, because
they are global applications.
- This was not taken into account by the database consistency checker
and were removed if the database consistency check was being run with
autofix enabled.
- Take into account to ignore these global OAuth2 applications when
running the database consistency check.
- Add unit tests.
- Ref: https://codeberg.org/Codeberg/Community/issues/1530

(cherry picked from commit 6af8f3a3f2)

models/auth/oauth2_test.go

@@ -4,11 +4,13 @@
 package auth_test
 
 import (
+	"path/filepath"
 	"testing"
 
 	auth_model "code.gitea.io/gitea/models/auth"
 	"code.gitea.io/gitea/models/db"
 	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/setting"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -265,3 +267,31 @@ func TestOAuth2AuthorizationCode_Invalidate(t *testing.T) {
 func TestOAuth2AuthorizationCode_TableName(t *testing.T) {
 	assert.Equal(t, "oauth2_authorization_code", new(auth_model.OAuth2AuthorizationCode).TableName())
 }
+
+func TestBuiltinApplicationsClientIDs(t *testing.T) {
+	assert.EqualValues(t, []string{"a4792ccc-144e-407e-86c9-5e7d8d9c3269", "e90ee53c-94e2-48ac-9358-a874fb9e0662", "d57cb8c4-630c-4168-8324-ec79935e18d4"}, auth_model.BuiltinApplicationsClientIDs())
+}
+
+func TestOrphanedOAuth2Applications(t *testing.T) {
+	defer unittest.OverrideFixtures(
+		unittest.FixturesOptions{
+			Dir:  filepath.Join(setting.AppWorkPath, "models/fixtures/"),
+			Base: setting.AppWorkPath,
+			Dirs: []string{"models/auth/TestOrphanedOAuth2Applications/"},
+		},
+	)()
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	count, err := auth_model.CountOrphanedOAuth2Applications(db.DefaultContext)
+	assert.NoError(t, err)
+	assert.EqualValues(t, 1, count)
+	unittest.AssertExistsIf(t, true, &auth_model.OAuth2Application{ID: 1002})
+
+	_, err = auth_model.DeleteOrphanedOAuth2Applications(db.DefaultContext)
+	assert.NoError(t, err)
+
+	count, err = auth_model.CountOrphanedOAuth2Applications(db.DefaultContext)
+	assert.NoError(t, err)
+	assert.EqualValues(t, 0, count)
+	unittest.AssertExistsIf(t, false, &auth_model.OAuth2Application{ID: 1002})
+}