fix: 补齐 Antigravity OAuth 账号 project_id 获取逻辑

部分账号 loadCodeAssist 不会立即返回 cloudaicompanionProject，
导致转发时 project 字段为空，上游返回 400 "Invalid project resource name projects/"。

- 新增 OnboardUser API：当 loadCodeAssist 未返回 project_id 时，
  通过 onboardUser 完成账号初始化并获取 project_id
- token 刷新时增加 onboard 兜底逻辑
- GetAccessToken 按需补齐：转发时发现 project_id 为空立即触发刷新
- 新增 resolveDefaultTierID 单元测试

backend/internal/service/antigravity_oauth_service.go

@@ -273,12 +273,20 @@ func (s *AntigravityOAuthService) loadProjectIDWithRetry(ctx context.Context, ac
 		}
 
 		client := antigravity.NewClient(proxyURL)
-		loadResp, _, err := client.LoadCodeAssist(ctx, accessToken)
+		loadResp, loadRaw, err := client.LoadCodeAssist(ctx, accessToken)
 
 		if err == nil && loadResp != nil && loadResp.CloudAICompanionProject != "" {
 			return loadResp.CloudAICompanionProject, nil
 		}
 
+		if err == nil {
+			if projectID, onboardErr := tryOnboardProjectID(ctx, client, accessToken, loadRaw); onboardErr == nil && projectID != "" {
+				return projectID, nil
+			} else if onboardErr != nil {
+				lastErr = onboardErr
+			}
+		}
+
 		// 记录错误
 		if err != nil {
 			lastErr = err
@@ -292,6 +300,53 @@ func (s *AntigravityOAuthService) loadProjectIDWithRetry(ctx context.Context, ac
 	return "", fmt.Errorf("获取 project_id 失败 (重试 %d 次后): %w", maxRetries, lastErr)
 }
 
+func tryOnboardProjectID(ctx context.Context, client *antigravity.Client, accessToken string, loadRaw map[string]any) (string, error) {
+	tierID := resolveDefaultTierID(loadRaw)
+	if tierID == "" {
+		return "", fmt.Errorf("loadCodeAssist 未返回可用的默认 tier")
+	}
+
+	projectID, err := client.OnboardUser(ctx, accessToken, tierID)
+	if err != nil {
+		return "", fmt.Errorf("onboardUser 失败 (tier=%s): %w", tierID, err)
+	}
+	return projectID, nil
+}
+
+func resolveDefaultTierID(loadRaw map[string]any) string {
+	if len(loadRaw) == 0 {
+		return ""
+	}
+
+	rawTiers, ok := loadRaw["allowedTiers"]
+	if !ok {
+		return ""
+	}
+
+	tiers, ok := rawTiers.([]any)
+	if !ok {
+		return ""
+	}
+
+	for _, rawTier := range tiers {
+		tier, ok := rawTier.(map[string]any)
+		if !ok {
+			continue
+		}
+		if isDefault, _ := tier["isDefault"].(bool); !isDefault {
+			continue
+		}
+		if id, ok := tier["id"].(string); ok {
+			id = strings.TrimSpace(id)
+			if id != "" {
+				return id
+			}
+		}
+	}
+
+	return ""
+}
+
 // BuildAccountCredentials 构建账户凭证
 func (s *AntigravityOAuthService) BuildAccountCredentials(tokenInfo *AntigravityTokenInfo) map[string]any {
 	creds := map[string]any{