fix(sora): 修复令牌刷新请求格式与流式错误转义

- 将 refresh_token 恢复请求改为表单编码并匹配 OAuth 约定
- 流式错误改为 JSON 序列化，避免消息含引号或换行导致 SSE 非法
- 补充 Sora token 恢复与 failover 流式错误透传回归测试

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

backend/internal/handler/sora_gateway_handler.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"crypto/sha256"
 	"encoding/hex"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -442,7 +443,18 @@ func (h *SoraGatewayHandler) handleStreamingAwareError(c *gin.Context, status in
 	if streamStarted {
 		flusher, ok := c.Writer.(http.Flusher)
 		if ok {
-			errorEvent := fmt.Sprintf(`event: error`+"\n"+`data: {"error": {"type": "%s", "message": "%s"}}`+"\n\n", errType, message)
+			errorData := map[string]any{
+				"error": map[string]string{
+					"type":    errType,
+					"message": message,
+				},
+			}
+			jsonBytes, err := json.Marshal(errorData)
+			if err != nil {
+				_ = c.Error(err)
+				return
+			}
+			errorEvent := fmt.Sprintf("event: error\ndata: %s\n\n", string(jsonBytes))
 			if _, err := fmt.Fprint(c.Writer, errorEvent); err != nil {
 				_ = c.Error(err)
 			}