feat: 完善 Antigravity 多平台网关支持，修复 Gemini handler 分流逻辑

backend/internal/service/antigravity_token_refresher.go

@@ -0,0 +1,57 @@
+package service
+
+import (
+	"context"
+	"strconv"
+	"time"
+)
+
+// AntigravityTokenRefresher 实现 TokenRefresher 接口
+type AntigravityTokenRefresher struct {
+	antigravityOAuthService *AntigravityOAuthService
+}
+
+func NewAntigravityTokenRefresher(antigravityOAuthService *AntigravityOAuthService) *AntigravityTokenRefresher {
+	return &AntigravityTokenRefresher{
+		antigravityOAuthService: antigravityOAuthService,
+	}
+}
+
+// CanRefresh 检查是否可以刷新此账户
+func (r *AntigravityTokenRefresher) CanRefresh(account *Account) bool {
+	return account.Platform == PlatformAntigravity && account.Type == AccountTypeOAuth
+}
+
+// NeedsRefresh 检查账户是否需要刷新
+func (r *AntigravityTokenRefresher) NeedsRefresh(account *Account, refreshWindow time.Duration) bool {
+	if !r.CanRefresh(account) {
+		return false
+	}
+	expiresAtStr := account.GetCredential("expires_at")
+	if expiresAtStr == "" {
+		return false
+	}
+	expiresAt, err := strconv.ParseInt(expiresAtStr, 10, 64)
+	if err != nil {
+		return false
+	}
+	expiryTime := time.Unix(expiresAt, 0)
+	return time.Until(expiryTime) < refreshWindow
+}
+
+// Refresh 执行 token 刷新
+func (r *AntigravityTokenRefresher) Refresh(ctx context.Context, account *Account) (map[string]any, error) {
+	tokenInfo, err := r.antigravityOAuthService.RefreshAccountToken(ctx, account)
+	if err != nil {
+		return nil, err
+	}
+
+	newCredentials := r.antigravityOAuthService.BuildAccountCredentials(tokenInfo)
+	for k, v := range account.Credentials {
+		if _, exists := newCredentials[k]; !exists {
+			newCredentials[k] = v
+		}
+	}
+
+	return newCredentials, nil
+}