refactor(gateway): remove parsed request object graphs

Keep large gateway payloads as raw body ranges and bind OpenAI parsed-body caches to the body bytes so failover and mapping do not reuse stale mutable state.

backend/internal/handler/gateway_handler.go

@@ -747,10 +747,10 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 			// 应用渠道模型映射到请求
 			if channelMapping.Mapped {
 				parsedReq.Model = channelMapping.MappedModel
-				parsedReq.Body.Replace(h.gatewayService.ReplaceModelInBody(parsedReq.Body.Bytes(), channelMapping.MappedModel))
+				parsedReq.ReplaceBody(h.gatewayService.ReplaceModelInBody(parsedReq.Body.Bytes(), channelMapping.MappedModel))
 			}
 			// Bedrock CC 兼容：渠道模型映射后，清理 Anthropic API 专有字段、注入 Bedrock 必需字段
-			parsedReq.Body.Replace(h.gatewayService.ApplyBedrockCCCompat(c.Request.Context(), parsedReq.Body.Bytes(), parsedReq.Model, account, apiKey.GroupID))
+			parsedReq.ReplaceBody(h.gatewayService.ApplyBedrockCCCompat(c.Request.Context(), parsedReq.Body.Bytes(), parsedReq.Model, account, apiKey.GroupID))
 			body = parsedReq.Body.Bytes()
 
 			// 转发请求 - 根据账号平台分流

backend/internal/handler/gateway_helper.go

@@ -74,8 +74,12 @@ func claudeCodeBodyMapFromParsedRequest(parsedReq *service.ParsedRequest) map[st
 	bodyMap := map[string]any{
 		"model": parsedReq.Model,
 	}
-	if parsedReq.System != nil || parsedReq.HasSystem {
-		bodyMap["system"] = parsedReq.System
+	if parsedReq.HasSystem {
+		if system, ok := parsedReq.SystemValue(); ok {
+			bodyMap["system"] = system
+		} else {
+			bodyMap["system"] = nil
+		}
 	}
 	if parsedReq.MetadataUserID != "" {
 		bodyMap["metadata"] = map[string]any{"user_id": parsedReq.MetadataUserID}
@@ -87,10 +91,8 @@ func claudeCodeBodyMapFromContextCache(c *gin.Context) map[string]any {
 	if c == nil {
 		return nil
 	}
-	if cached, ok := c.Get(service.OpenAIParsedRequestBodyKey); ok {
-		if bodyMap, ok := cached.(map[string]any); ok {
-			return bodyMap
-		}
+	if bodyMap := service.CachedOpenAIParsedRequestBody(c); bodyMap != nil {
+		return bodyMap
 	}
 	if cached, ok := c.Get(claudeCodeParsedRequestContextKey); ok {
 		switch v := cached.(type) {

backend/internal/handler/gateway_helper_hotpath_test.go

@@ -185,13 +185,8 @@ func TestSetClaudeCodeClientContext_ReuseParsedRequestAndContextCache(t *testing
 		c.Request.Header.Set("anthropic-beta", "message-batches-2024-09-24")
 		c.Request.Header.Set("anthropic-version", "2023-06-01")
 
-		parsedReq := &service.ParsedRequest{
-			Model: "claude-3-5-sonnet-20241022",
-			System: []any{
-				map[string]any{"text": "You are Claude Code, Anthropic's official CLI for Claude."},
-			},
-			MetadataUserID: "user_aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa_account__session_aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa",
-		}
+		parsedReq, err := service.ParseGatewayRequest(service.NewRequestBodyRef(validClaudeCodeBodyJSON()), "")
+		require.NoError(t, err)
 
 		// body 非法 JSON，如果函数复用 parsedReq 成功则仍应判定为 Claude Code。
 		SetClaudeCodeClientContext(c, []byte(`{invalid`), parsedReq)
@@ -204,7 +199,7 @@ func TestSetClaudeCodeClientContext_ReuseParsedRequestAndContextCache(t *testing
 		c.Request.Header.Set("X-App", "claude-code")
 		c.Request.Header.Set("anthropic-beta", "message-batches-2024-09-24")
 		c.Request.Header.Set("anthropic-version", "2023-06-01")
-		c.Set(service.OpenAIParsedRequestBodyKey, map[string]any{
+		service.CacheOpenAIParsedRequestBody(c, []byte(`{invalid`), map[string]any{
 			"model": "claude-3-5-sonnet-20241022",
 			"system": []any{
 				map[string]any{"text": "You are Claude Code, Anthropic's official CLI for Claude."},

backend/internal/handler/openai_gateway_handler.go

@@ -954,7 +954,7 @@ func (h *OpenAIGatewayHandler) validateFunctionCallOutputRequest(c *gin.Context,
 		return true
 	}
 
-	c.Set(service.OpenAIParsedRequestBodyKey, reqBody)
+	service.CacheOpenAIParsedRequestBody(c, body, reqBody)
 	validation := service.ValidateFunctionCallOutputContext(reqBody)
 	if !validation.HasFunctionCallOutput {
 		return true