fix(gateway): 修复 base_url 包含 /chat/completions 时路径拼接错误

问题：
- 当账号的 base_url 配置为 https://example.com/v1/chat/completions 时
- 代码直接追加 /responses，导致路径变成 /v1/chat/completions/responses
- 上游返回 404 错误

修复：
- 在追加 /responses 前，先移除 base_url 中的 /chat/completions 后缀
- 确保最终路径为 https://example.com/v1/responses

影响范围：
- OpenAI API Key 账号的测试接口
- OpenAI API Key 账号的实际网关请求

Related-to: #231

backend/internal/service/account_test_service.go

@@ -332,7 +332,10 @@ func (s *AccountTestService) testOpenAIAccountConnection(c *gin.Context, account
 		if err != nil {
 			return s.sendErrorAndEnd(c, fmt.Sprintf("Invalid base URL: %s", err.Error()))
 		}
-		apiURL = strings.TrimSuffix(normalizedBaseURL, "/") + "/responses"
+		// Remove /chat/completions suffix if present, then add /responses
+		normalizedBaseURL = strings.TrimSuffix(normalizedBaseURL, "/")
+		normalizedBaseURL = strings.TrimSuffix(normalizedBaseURL, "/chat/completions")
+		apiURL = normalizedBaseURL + "/responses"
 	} else {
 		return s.sendErrorAndEnd(c, fmt.Sprintf("Unsupported account type: %s", account.Type))
 	}

backend/internal/service/openai_gateway_service.go

@@ -740,6 +740,9 @@ func (s *OpenAIGatewayService) buildUpstreamRequest(ctx context.Context, c *gin.
 			if err != nil {
 				return nil, err
 			}
+			// Remove /chat/completions suffix if present, then add /responses
+			validatedURL = strings.TrimSuffix(validatedURL, "/")
+			validatedURL = strings.TrimSuffix(validatedURL, "/chat/completions")
 			targetURL = validatedURL + "/responses"
 		}
 	default: