refactor: 去掉 GetDerivatives 的假 error 返回 + 同步 API 文档

GetDerivatives 签名里的 error 返回永远是 nil，controller 那段 if err != nil
是死分支，会误导调用方以为存在"全失败"语义。当前产品策略是 best-effort
response：局部失败 → warnings，bundle 永远非 nil，HTTP 永远 200。
等 Hermes 上游真要求"全失败 → 5xx"时再重新引入 error 返回。

同时把 docs/dev.md §13.5 示例补上 warnings 字段（上次重构遗漏的文档同步）。

internal/controller/restapi/v1/market_routes.go

@@ -76,10 +76,7 @@ func RegisterMarket(r fiber.Router, d MarketDeps) {
 			return fiber.NewError(fiber.StatusBadRequest, "symbol is required")
 		}
 
-		bundle, warnings, err := d.MarketQuery.GetDerivatives(c.UserContext(), symbol, period)
-		if err != nil {
-			return fiber.NewError(fiber.StatusInternalServerError, err.Error())
-		}
+		bundle, warnings := d.MarketQuery.GetDerivatives(c.UserContext(), symbol, period)
 
 		return c.JSON(fiber.Map{
 			"symbol":             symbol,

internal/usecase/market_query.go

@@ -36,7 +36,7 @@ func NewMarketQueryUsecase(
 	}
 }
 
-func (u *MarketQueryUsecase) GetDerivatives(ctx context.Context, symbol, period string) (*entity.DerivativesBundle, []string, error) {
+func (u *MarketQueryUsecase) GetDerivatives(ctx context.Context, symbol, period string) (*entity.DerivativesBundle, []string) {
 	var (
 		currentFund *entity.FundingRate
 		currentOI   *entity.OpenInterest
@@ -109,5 +109,5 @@ func (u *MarketQueryUsecase) GetDerivatives(ctx context.Context, symbol, period
 	if warnings == nil {
 		warnings = []string{}
 	}
-	return bundle, warnings, nil
+	return bundle, warnings
 }