refactor: extract ReadUpstreamResponseBody to deduplicate upstream response read + too-large error handling

Consolidates 9 call sites of resolveUpstreamResponseReadLimit + readUpstreamResponseBodyLimited + ErrUpstreamResponseBodyTooLarge error handling into a single ReadUpstreamResponseBody function with TooLargeWriter callback for API-format-specific error responses (Anthropic, OpenAI, countTokens).

backend/internal/service/upstream_response_limit.go

@@ -4,8 +4,10 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"net/http"
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/gin-gonic/gin"
 )
 
 var ErrUpstreamResponseBodyTooLarge = errors.New("upstream response body too large")
@@ -36,3 +38,44 @@ func readUpstreamResponseBodyLimited(reader io.Reader, maxBytes int64) ([]byte,
 	}
 	return body, nil
 }
+
+// TooLargeWriter 在响应超限时向客户端写格式化的错误响应。
+type TooLargeWriter func(c *gin.Context)
+
+// ReadUpstreamResponseBody 读取上游非流式响应体。
+// 超限时自动记录 ops error 并调用 onTooLarge 向客户端写错误。
+func ReadUpstreamResponseBody(reader io.Reader, cfg *config.Config, c *gin.Context, onTooLarge TooLargeWriter) ([]byte, error) {
+	maxBytes := resolveUpstreamResponseReadLimit(cfg)
+	body, err := readUpstreamResponseBodyLimited(reader, maxBytes)
+	if err != nil {
+		if errors.Is(err, ErrUpstreamResponseBodyTooLarge) {
+			setOpsUpstreamError(c, http.StatusBadGateway, "upstream response too large", "")
+			if onTooLarge != nil {
+				onTooLarge(c)
+			}
+		}
+		return nil, err
+	}
+	return body, nil
+}
+
+// anthropicTooLargeError 以 Anthropic Messages API 格式写入超限错误。
+func anthropicTooLargeError(c *gin.Context) {
+	c.JSON(http.StatusBadGateway, gin.H{
+		"type": "error",
+		"error": gin.H{
+			"type":    "upstream_error",
+			"message": "Upstream response too large",
+		},
+	})
+}
+
+// openAITooLargeError 以 OpenAI / Gemini 格式写入超限错误。
+func openAITooLargeError(c *gin.Context) {
+	c.JSON(http.StatusBadGateway, gin.H{
+		"error": gin.H{
+			"type":    "upstream_error",
+			"message": "Upstream response too large",
+		},
+	})
+}