refactor: remove unused ClientSecret constant

The ClientSecret constant was left as an empty string after
getClientSecret() was refactored to use defaultClientSecret.
Remove the dead constant and update the test accordingly.

backend/internal/pkg/antigravity/oauth.go

@@ -24,10 +24,8 @@ const (
 
 	// Antigravity OAuth 客户端凭证
 	ClientID = "1071006060591-tmhssin2h21lcre235vtolojh4g403ep.apps.googleusercontent.com"
-	ClientSecret = ""
 
 	// AntigravityOAuthClientSecretEnv 是 Antigravity OAuth client_secret 的环境变量名。
-	// 出于安全原因，该值不得硬编码入库。
 	AntigravityOAuthClientSecretEnv = "ANTIGRAVITY_OAUTH_CLIENT_SECRET"
 
 	// 固定的 redirect_uri（用户需手动复制 code）

backend/internal/pkg/antigravity/oauth_test.go

@@ -680,9 +680,6 @@ func TestConstants_值正确(t *testing.T) {
 	if ClientID != "1071006060591-tmhssin2h21lcre235vtolojh4g403ep.apps.googleusercontent.com" {
 		t.Errorf("ClientID 不匹配: got %s", ClientID)
 	}
-	if ClientSecret != "" {
-		t.Error("ClientSecret 常量应为空字符串（默认值已移至 defaultClientSecret）")
-	}
 	secret, err := getClientSecret()
 	if err != nil {
 		t.Fatalf("getClientSecret 应返回默认值，但报错: %v", err)