refactor: move channel model restriction from handler to scheduling phase

Move the model pricing restriction check from 8 handler entry points
to the account scheduling phase (SelectAccountForModelWithExclusions /
SelectAccountWithLoadAwareness), aligning restriction with billing:

- requested: check original request model against pricing list
- channel_mapped: check channel-mapped model against pricing list
- upstream: per-account check using account-mapped model

Handler layer now only resolves channel mapping (no restriction).
Scheduling layer performs pre-check for requested/channel_mapped,
and per-account filtering for upstream billing source.

backend/internal/handler/openai_gateway_handler.go

@@ -47,13 +47,6 @@ func resolveOpenAIForwardDefaultMappedModel(apiKey *service.APIKey, fallbackMode
 	return strings.TrimSpace(apiKey.Group.DefaultMappedModel)
 }
 
-func resolveOpenAIMessagesDispatchMappedModel(apiKey *service.APIKey, requestedModel string) string {
-	if apiKey == nil || apiKey.Group == nil {
-		return ""
-	}
-	return strings.TrimSpace(apiKey.Group.ResolveMessagesDispatchModel(requestedModel))
-}
-
 // NewOpenAIGatewayHandler creates a new OpenAIGatewayHandler
 func NewOpenAIGatewayHandler(
 	gatewayService *service.OpenAIGatewayService,
@@ -557,8 +550,6 @@ func (h *OpenAIGatewayHandler) Messages(c *gin.Context) {
 		return
 	}
 	reqModel := modelResult.String()
-	routingModel := service.NormalizeOpenAICompatRequestedModel(reqModel)
-	preferredMappedModel := resolveOpenAIMessagesDispatchMappedModel(apiKey, reqModel)
 	reqStream := gjson.GetBytes(body, "stream").Bool()
 
 	reqLog = reqLog.With(zap.String("model", reqModel), zap.Bool("stream", reqStream))
@@ -617,20 +608,17 @@ func (h *OpenAIGatewayHandler) Messages(c *gin.Context) {
 	failedAccountIDs := make(map[int64]struct{})
 	sameAccountRetryCount := make(map[int64]int)
 	var lastFailoverErr *service.UpstreamFailoverError
-	effectiveMappedModel := preferredMappedModel
 
 	for {
-		currentRoutingModel := routingModel
-		if effectiveMappedModel != "" {
-			currentRoutingModel = effectiveMappedModel
-		}
+		// 清除上一次迭代的降级模型标记，避免残留影响本次迭代
+		c.Set("openai_messages_fallback_model", "")
 		reqLog.Debug("openai_messages.account_selecting", zap.Int("excluded_account_count", len(failedAccountIDs)))
 		selection, scheduleDecision, err := h.gatewayService.SelectAccountWithScheduler(
 			c.Request.Context(),
 			apiKey.GroupID,
 			"", // no previous_response_id
 			sessionHash,
-			currentRoutingModel,
+			reqModel,
 			failedAccountIDs,
 			service.OpenAIUpstreamTransportAny,
 		)
@@ -639,7 +627,29 @@ func (h *OpenAIGatewayHandler) Messages(c *gin.Context) {
 				zap.Error(err),
 				zap.Int("excluded_account_count", len(failedAccountIDs)),
 			)
+			// 首次调度失败 + 有默认映射模型 → 用默认模型重试
 			if len(failedAccountIDs) == 0 {
+				defaultModel := ""
+				if apiKey.Group != nil {
+					defaultModel = apiKey.Group.DefaultMappedModel
+				}
+				if defaultModel != "" && defaultModel != reqModel {
+					reqLog.Info("openai_messages.fallback_to_default_model",
+						zap.String("default_mapped_model", defaultModel),
+					)
+					selection, scheduleDecision, err = h.gatewayService.SelectAccountWithScheduler(
+						c.Request.Context(),
+						apiKey.GroupID,
+						"",
+						sessionHash,
+						defaultModel,
+						failedAccountIDs,
+						service.OpenAIUpstreamTransportAny,
+					)
+					if err == nil && selection != nil {
+						c.Set("openai_messages_fallback_model", defaultModel)
+					}
+				}
 				if err != nil {
 					h.anthropicStreamingAwareError(c, http.StatusServiceUnavailable, "api_error", "Service temporarily unavailable", streamStarted)
 					return
@@ -671,7 +681,9 @@ func (h *OpenAIGatewayHandler) Messages(c *gin.Context) {
 		service.SetOpsLatencyMs(c, service.OpsRoutingLatencyMsKey, time.Since(routingStart).Milliseconds())
 		forwardStart := time.Now()
 
-		defaultMappedModel := strings.TrimSpace(effectiveMappedModel)
+		// Forward 层需要始终拿到 group 默认映射模型，这样未命中账号级映射的
+		// Claude 兼容模型才不会在后续 Codex 规范化中意外退化到 gpt-5.1。
+		defaultMappedModel := resolveOpenAIForwardDefaultMappedModel(apiKey, c.GetString("openai_messages_fallback_model"))
 		// 应用渠道模型映射到请求体
 		forwardBody := body
 		if channelMappingMsg.Mapped {
@@ -1106,7 +1118,7 @@ func (h *OpenAIGatewayHandler) ResponsesWebSocket(c *gin.Context) {
 	setOpsRequestContext(c, reqModel, true, firstMessage)
 	setOpsEndpointContext(c, "", int16(service.RequestTypeWSV2))
 
-	// 解析渠道级模型映射
+	// 解析渠道级模型映射 + 限制检查
 	channelMappingWS, _ := h.gatewayService.ResolveChannelMappingAndRestrict(ctx, apiKey.GroupID, reqModel)
 
 	var currentUserRelease func()