fix(gateway): 明确旧协议接口不支持的错误提示

将 /v1/chat/completions 的拦截文案改为旧协议不支持，避免误导为会路由到 Sora。
明确要求客户端改用 /v1/responses。

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

backend/internal/server/routes/gateway.go

@@ -43,12 +43,12 @@ func RegisterGatewayRoutes(
 		gateway.GET("/usage", h.Gateway.Usage)
 		// OpenAI Responses API
 		gateway.POST("/responses", h.OpenAIGateway.Responses)
-		// 明确阻止旧入口误用到 Sora，避免客户端把 OpenAI Chat Completions 当作 Sora 入口
+		// 明确阻止旧协议入口：OpenAI 仅支持 Responses API，避免客户端误解为会自动路由到其它平台。
 		gateway.POST("/chat/completions", func(c *gin.Context) {
 			c.JSON(http.StatusBadRequest, gin.H{
 				"error": gin.H{
 					"type":    "invalid_request_error",
-					"message": "For Sora, use /sora/v1/chat/completions. OpenAI should use /v1/responses.",
+					"message": "Unsupported legacy protocol: /v1/chat/completions is not supported. Please use /v1/responses.",
 				},
 			})
 		})