fix(gateway): 完善 max_output_tokens 参数处理逻辑

根据不同平台和账号类型处理 max_output_tokens 参数：
- OpenAI OAuth (Responses API): 保留 max_output_tokens（支持）
- OpenAI API Key: 删除 max_output_tokens（不支持）
- Anthropic (Claude): 转换 max_output_tokens 为 max_tokens
- Gemini: 删除 max_output_tokens（由 Gemini 专用转换处理）
- 其他平台: 删除（安全起见）

同时处理 max_completion_tokens 参数，仅在 OpenAI OAuth 时保留。

修复客户端（如 OpenCode）发送不支持参数导致上游返回 400 错误的问题。

Related-to: #231

backend/internal/service/openai_gateway_service.go

@@ -568,6 +568,44 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 		}
 	}
 
+	// Handle max_output_tokens based on platform and account type
+	if !isCodexCLI {
+		if maxOutputTokens, hasMaxOutputTokens := reqBody["max_output_tokens"]; hasMaxOutputTokens {
+			switch account.Platform {
+			case PlatformOpenAI:
+				// For OpenAI API Key, remove max_output_tokens (not supported)
+				// For OpenAI OAuth (Responses API), keep it (supported)
+				if account.Type == AccountTypeAPIKey {
+					delete(reqBody, "max_output_tokens")
+					bodyModified = true
+				}
+			case PlatformAnthropic:
+				// For Anthropic (Claude), convert to max_tokens
+				delete(reqBody, "max_output_tokens")
+				if _, hasMaxTokens := reqBody["max_tokens"]; !hasMaxTokens {
+					reqBody["max_tokens"] = maxOutputTokens
+				}
+				bodyModified = true
+			case PlatformGemini:
+				// For Gemini, remove (will be handled by Gemini-specific transform)
+				delete(reqBody, "max_output_tokens")
+				bodyModified = true
+			default:
+				// For unknown platforms, remove to be safe
+				delete(reqBody, "max_output_tokens")
+				bodyModified = true
+			}
+		}
+
+		// Also handle max_completion_tokens (similar logic)
+		if _, hasMaxCompletionTokens := reqBody["max_completion_tokens"]; hasMaxCompletionTokens {
+			if account.Type == AccountTypeAPIKey || account.Platform != PlatformOpenAI {
+				delete(reqBody, "max_completion_tokens")
+				bodyModified = true
+			}
+		}
+	}
+
 	// Re-serialize body only if modified
 	if bodyModified {
 		var err error