fix: OpenAI临时性400错误支持池模式同账号重试 & HelpTooltip层级修复

1. 识别OpenAI "An error occurred while processing your request" 临时性400错误
   并触发failover，同时在池模式下标记RetryableOnSameAccount，允许同账号重试
2. ForwardAsAnthropic路径同步支持临时性400错误的识别和同账号重试
3. HelpTooltip组件使用Teleport渲染到body，修复在dialog内被裁切的问题

backend/internal/service/openai_gateway_messages.go

@@ -2,6 +2,7 @@ package service
 
 import (
 	"bufio"
+	"bytes"
 	"context"
 	"encoding/json"
 	"errors"
@@ -140,12 +141,13 @@ func (s *OpenAIGatewayService) ForwardAsAnthropic(
 
 	// 8. Handle error response with failover
 	if resp.StatusCode >= 400 {
-		if s.shouldFailoverUpstreamError(resp.StatusCode) {
-			respBody, _ := io.ReadAll(io.LimitReader(resp.Body, 2<<20))
-			_ = resp.Body.Close()
+		respBody, _ := io.ReadAll(io.LimitReader(resp.Body, 2<<20))
+		_ = resp.Body.Close()
+		resp.Body = io.NopCloser(bytes.NewReader(respBody))
 
-			upstreamMsg := strings.TrimSpace(extractUpstreamErrorMessage(respBody))
-			upstreamMsg = sanitizeUpstreamErrorMessage(upstreamMsg)
+		upstreamMsg := strings.TrimSpace(extractUpstreamErrorMessage(respBody))
+		upstreamMsg = sanitizeUpstreamErrorMessage(upstreamMsg)
+		if s.shouldFailoverOpenAIUpstreamResponse(resp.StatusCode, upstreamMsg, respBody) {
 			upstreamDetail := ""
 			if s.cfg != nil && s.cfg.Gateway.LogUpstreamErrorBody {
 				maxBytes := s.cfg.Gateway.LogUpstreamErrorBodyMaxBytes
@@ -167,7 +169,11 @@ func (s *OpenAIGatewayService) ForwardAsAnthropic(
 			if s.rateLimitService != nil {
 				s.rateLimitService.HandleUpstreamError(ctx, account, resp.StatusCode, resp.Header, respBody)
 			}
-			return nil, &UpstreamFailoverError{StatusCode: resp.StatusCode, ResponseBody: respBody}
+			return nil, &UpstreamFailoverError{
+				StatusCode:             resp.StatusCode,
+				ResponseBody:           respBody,
+				RetryableOnSameAccount: account.IsPoolMode() && isOpenAITransientProcessingError(resp.StatusCode, upstreamMsg, respBody),
+			}
 		}
 		// Non-failover error: return Anthropic-formatted error to client
 		return s.handleAnthropicErrorResponse(resp, c, account)