fix(认证): 修复 OAuth token 缓存失效与 401 处理

新增 token 缓存失效接口并在刷新后清理
401 限流支持自定义规则与可配置冷却时间
补齐缓存失效与 401 处理测试
测试: make test

backend/internal/service/gemini_token_cache.go

@@ -10,6 +10,7 @@ type GeminiTokenCache interface {
 	// cacheKey should be stable for the token scope; for GeminiCli OAuth we primarily use project_id.
 	GetAccessToken(ctx context.Context, cacheKey string) (string, error)
 	SetAccessToken(ctx context.Context, cacheKey string, token string, ttl time.Duration) error
+	DeleteAccessToken(ctx context.Context, cacheKey string) error
 
 	AcquireRefreshLock(ctx context.Context, cacheKey string, ttl time.Duration) (bool, error)
 	ReleaseRefreshLock(ctx context.Context, cacheKey string) error