fix: Antigravity 刷新 token 时检测 project_id 缺失

- 刷新 token 后调用 LoadCodeAssist 获取 project_id
- 如果获取失败，保留原有 project_id，标记账户为 error
- token 仍会正常更新，不影响凭证刷新
- 错误信息：账户缺少project id，可能无法使用Antigravity

backend/internal/service/antigravity_oauth_service.go

@@ -82,13 +82,14 @@ type AntigravityExchangeCodeInput struct {
 
 // AntigravityTokenInfo token 信息
 type AntigravityTokenInfo struct {
-	AccessToken  string `json:"access_token"`
-	RefreshToken string `json:"refresh_token"`
-	ExpiresIn    int64  `json:"expires_in"`
-	ExpiresAt    int64  `json:"expires_at"`
-	TokenType    string `json:"token_type"`
-	Email        string `json:"email,omitempty"`
-	ProjectID    string `json:"project_id,omitempty"`
+	AccessToken      string `json:"access_token"`
+	RefreshToken     string `json:"refresh_token"`
+	ExpiresIn        int64  `json:"expires_in"`
+	ExpiresAt        int64  `json:"expires_at"`
+	TokenType        string `json:"token_type"`
+	Email            string `json:"email,omitempty"`
+	ProjectID        string `json:"project_id,omitempty"`
+	ProjectIDMissing bool   `json:"-"` // LoadCodeAssist 未返回 project_id
 }
 
 // ExchangeCode 用 authorization code 交换 token
@@ -236,16 +237,15 @@ func (s *AntigravityOAuthService) RefreshAccountToken(ctx context.Context, accou
 		tokenInfo.Email = existingEmail
 	}
 
-	// 每次刷新都调用 LoadCodeAssist 更新 project_id
+	// 每次刷新都调用 LoadCodeAssist 获取 project_id
 	client := antigravity.NewClient(proxyURL)
 	loadResp, _, err := client.LoadCodeAssist(ctx, tokenInfo.AccessToken)
-	if err != nil {
-		// 失败时保留原有的 project_id
+	if err != nil || loadResp == nil || loadResp.CloudAICompanionProject == "" {
+		// LoadCodeAssist 失败或返回空，保留原有 project_id，标记缺失
 		existingProjectID := strings.TrimSpace(account.GetCredential("project_id"))
-		if existingProjectID != "" {
-			tokenInfo.ProjectID = existingProjectID
-		}
-	} else if loadResp != nil && loadResp.CloudAICompanionProject != "" {
+		tokenInfo.ProjectID = existingProjectID
+		tokenInfo.ProjectIDMissing = true
+	} else {
 		tokenInfo.ProjectID = loadResp.CloudAICompanionProject
 	}