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/gemini_v1beta_handler.go

@@ -121,7 +121,7 @@ func (h *GatewayHandler) GeminiV1BetaGetModel(c *gin.Context) {
 		googleError(c, http.StatusBadGateway, err.Error())
 		return
 	}
-	if shouldFallbackGeminiModel(modelName, res) {
+	if shouldFallbackGeminiModels(res) {
 		c.JSON(http.StatusOK, gemini.FallbackModel(modelName))
 		return
 	}
@@ -184,7 +184,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 	setOpsRequestContext(c, modelName, stream, body)
 	setOpsEndpointContext(c, "", int16(service.RequestTypeFromLegacy(stream, false)))
 
-	// 解析渠道级模型映射
+	// 解析渠道级模型映射 + 限制检查
 	channelMapping, _ := h.gatewayService.ResolveChannelMappingAndRestrict(c.Request.Context(), apiKey.GroupID, modelName)
 	reqModel := modelName // 保存映射前的原始模型名
 	if channelMapping.Mapped {
@@ -682,16 +682,6 @@ func shouldFallbackGeminiModels(res *service.UpstreamHTTPResult) bool {
 	return false
 }
 
-func shouldFallbackGeminiModel(modelName string, res *service.UpstreamHTTPResult) bool {
-	if shouldFallbackGeminiModels(res) {
-		return true
-	}
-	if res == nil || res.StatusCode != http.StatusNotFound {
-		return false
-	}
-	return gemini.HasFallbackModel(modelName)
-}
-
 // extractGeminiCLISessionHash 从 Gemini CLI 请求中提取会话标识。
 // 组合 x-gemini-api-privileged-user-id header 和请求体中的 tmp 目录哈希。
 //