enhance(oauth): pre-register a builtin OAuth2 application for gitea-vscode (#39785)

The [Gitea VS Code extension](https://gitea.com/gitea/gitea-vscode)
currently signs in through the builtin `tea` OAuth2 application. Gitea
keeps one grant per user and application, so anyone who has already
signed in with the tea CLI gets `a grant exists with different scope`
when the extension requests its own scopes, and sign-in fails
(gitea/gitea-vscode#24).

This adds a dedicated builtin application for the extension, following
the same mechanism as the existing CLI entries:

modules/setting/oauth2.go

@@ -108,7 +108,7 @@ var OAuth2 = struct {
 	JWTSigningAlgorithm:        "RS256",
 	JWTSigningPrivateKeyFile:   "jwt/private.pem",
 	MaxTokenLength:             math.MaxInt16,
-	DefaultApplications:        []string{"git-credential-oauth", "git-credential-manager", "tea", "gitea-app"},
+	DefaultApplications:        []string{"git-credential-oauth", "git-credential-manager", "tea", "gitea-app", "gitea-vscode"},
 }
 
 func loadOAuth2From(rootCfg ConfigProvider) {

modules/setting/oauth2_test.go

@@ -62,7 +62,7 @@ func TestGetGeneralSigningSecretSave(t *testing.T) {
 func TestOauth2DefaultApplications(t *testing.T) {
 	cfg, _ := NewConfigProviderFromData(``)
 	loadOAuth2From(cfg)
-	assert.Equal(t, []string{"git-credential-oauth", "git-credential-manager", "tea", "gitea-app"}, OAuth2.DefaultApplications)
+	assert.Equal(t, []string{"git-credential-oauth", "git-credential-manager", "tea", "gitea-app", "gitea-vscode"}, OAuth2.DefaultApplications)
 
 	cfg, _ = NewConfigProviderFromData(`[oauth2]
 DEFAULT_APPLICATIONS = tea