refactor(service): 统一时间戳解析，支持多种格式

新增 Account.GetCredentialAsTime 方法，统一处理凭证中的时间戳字段，
兼容 RFC3339 字符串、Unix 时间戳字符串和数字类型。

- 重构 Claude/Gemini/Antigravity TokenRefresher.NeedsRefresh
- 移除重复的 parseExpiresAt/parseAntigravityExpiresAt 函数
- 简化 GetOpenAITokenExpiresAt 实现
- 新增 RFC3339 格式单元测试用例

backend/internal/service/gemini_token_provider.go

@@ -50,7 +50,7 @@ func (p *GeminiTokenProvider) GetAccessToken(ctx context.Context, account *Accou
 	}
 
 	// 2) Refresh if needed (pre-expiry skew).
-	expiresAt := parseExpiresAt(account)
+	expiresAt := account.GetCredentialAsTime("expires_at")
 	needsRefresh := expiresAt == nil || time.Until(*expiresAt) <= geminiTokenRefreshSkew
 	if needsRefresh && p.tokenCache != nil {
 		locked, err := p.tokenCache.AcquireRefreshLock(ctx, cacheKey, 30*time.Second)
@@ -66,7 +66,7 @@ func (p *GeminiTokenProvider) GetAccessToken(ctx context.Context, account *Accou
 			if err == nil && fresh != nil {
 				account = fresh
 			}
-			expiresAt = parseExpiresAt(account)
+			expiresAt = account.GetCredentialAsTime("expires_at")
 			if expiresAt == nil || time.Until(*expiresAt) <= geminiTokenRefreshSkew {
 				if p.geminiOAuthService == nil {
 					return "", errors.New("gemini oauth service not configured")
@@ -83,7 +83,7 @@ func (p *GeminiTokenProvider) GetAccessToken(ctx context.Context, account *Accou
 				}
 				account.Credentials = newCredentials
 				_ = p.accountRepo.Update(ctx, account)
-				expiresAt = parseExpiresAt(account)
+				expiresAt = account.GetCredentialAsTime("expires_at")
 			}
 		}
 	}
@@ -154,18 +154,3 @@ func geminiTokenCacheKey(account *Account) string {
 	}
 	return "account:" + strconv.FormatInt(account.ID, 10)
 }
-
-func parseExpiresAt(account *Account) *time.Time {
-	raw := strings.TrimSpace(account.GetCredential("expires_at"))
-	if raw == "" {
-		return nil
-	}
-	if unixSec, err := strconv.ParseInt(raw, 10, 64); err == nil && unixSec > 0 {
-		t := time.Unix(unixSec, 0)
-		return &t
-	}
-	if t, err := time.Parse(time.RFC3339, raw); err == nil {
-		return &t
-	}
-	return nil
-}