perf(service): 优化 model 替换函数，用 gjson/sjson 替代全量 JSON 序列化

SSE 热路径中 replaceModelInSSELine 和 replaceModelInResponseBody 原来
使用 json.Unmarshal/Marshal 对每个事件做全量反序列化再序列化，现改为
gjson.Get/sjson.Set 精确字段操作，消除 O(n) 中间 map 分配，保持 JSON
字段顺序不变。涉及 OpenAIGatewayService 和 GatewayService 两个服务。

新增 23 个单元测试覆盖：顶层/嵌套 model 替换、不匹配跳过、空行/[DONE]/
非法 JSON 等边界情况。

Fixes: P1-08

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/openai_gateway_service.go

@@ -24,6 +24,8 @@ import (
 	"github.com/Wei-Shaw/sub2api/internal/util/responseheaders"
 	"github.com/Wei-Shaw/sub2api/internal/util/urlvalidator"
 	"github.com/gin-gonic/gin"
+	"github.com/tidwall/gjson"
+	"github.com/tidwall/sjson"
 )
 
 const (
@@ -1430,31 +1432,22 @@ func (s *OpenAIGatewayService) replaceModelInSSELine(line, fromModel, toModel st
 		return line
 	}
 
-	var event map[string]any
-	if err := json.Unmarshal([]byte(data), &event); err != nil {
-		return line
-	}
-
-	// Replace model in response
-	if m, ok := event["model"].(string); ok && m == fromModel {
-		event["model"] = toModel
-		newData, err := json.Marshal(event)
+	// 使用 gjson 精确检查 model 字段，避免全量 JSON 反序列化
+	if m := gjson.Get(data, "model"); m.Exists() && m.Str == fromModel {
+		newData, err := sjson.Set(data, "model", toModel)
 		if err != nil {
 			return line
 		}
-		return "data: " + string(newData)
+		return "data: " + newData
 	}
 
-	// Check nested response
-	if response, ok := event["response"].(map[string]any); ok {
-		if m, ok := response["model"].(string); ok && m == fromModel {
-			response["model"] = toModel
-			newData, err := json.Marshal(event)
-			if err != nil {
-				return line
-			}
-			return "data: " + string(newData)
+	// 检查嵌套的 response.model 字段
+	if m := gjson.Get(data, "response.model"); m.Exists() && m.Str == fromModel {
+		newData, err := sjson.Set(data, "response.model", toModel)
+		if err != nil {
+			return line
 		}
+		return "data: " + newData
 	}
 
 	return line
@@ -1674,23 +1667,15 @@ func (s *OpenAIGatewayService) validateUpstreamBaseURL(raw string) (string, erro
 }
 
 func (s *OpenAIGatewayService) replaceModelInResponseBody(body []byte, fromModel, toModel string) []byte {
-	var resp map[string]any
-	if err := json.Unmarshal(body, &resp); err != nil {
-		return body
+	// 使用 gjson/sjson 精确替换 model 字段，避免全量 JSON 反序列化
+	if m := gjson.GetBytes(body, "model"); m.Exists() && m.Str == fromModel {
+		newBody, err := sjson.SetBytes(body, "model", toModel)
+		if err != nil {
+			return body
+		}
+		return newBody
 	}
-
-	model, ok := resp["model"].(string)
-	if !ok || model != fromModel {
-		return body
-	}
-
-	resp["model"] = toModel
-	newBody, err := json.Marshal(resp)
-	if err != nil {
-		return body
-	}
-
-	return newBody
+	return body
 }
 
 // OpenAIRecordUsageInput input for recording usage