AWS Lambda执行Stack Exchange OAuth PKCE流程遇Cloudflare验证问题

我在AWS Lambda函数中实现Stack Exchange OAuth 2.0 PKCE流程，用完全相同的参数在Postman测试时，token交换能正常完成，但通过Lambda调用却失败了。

Lambda函数代码如下：

export const handler = async (event) => {
  try {
    const body = typeof event.body === "string" ? JSON.parse(event.body) : event.body ?? {};
    
    const code = body.code;
    const codeVerifier = body.code_verifier;
    const redirectUri = body.redirect_uri;
   
    
    const params = new URLSearchParams({
      client_id: String(process.env.STACKEXCHANGE_CLIENT_ID),
      client_secret: String(process.env.STACKEXCHANGE_CLIENT_SECRET),
      redirect_uri: redirectUri,
      code_verifier: codeVerifier,
      code: code
    });

    const tokenResponse = await fetch("https://stackoverflow.com/oauth/access_token/json", {
      method: "POST",
      headers: {
        'Content-Type': 'application/x-www-form-urlencoded',
        'Accept': 'application/json',
      },
      body: params.toString()
    });

    const contentType = tokenResponse.headers.get("content-type");
    
    if (contentType && contentType.includes("application/json")) {
      const data = await tokenResponse.json();
      return response(200, { access_token: data.access_token });
    } else {
      const text = await tokenResponse.text();
      console.error("Non-JSON response:", text);
      return response(tokenResponse.status, {
        error: "Non-JSON response from StackOverflow"
      });
    }
  } catch (err) {
    return response(500, { error: err.message });
  }
};

调用Lambda时，没有收到预期的JSON响应，反而得到一个Cloudflare机器人防护验证页面，页面开头内容如下：

<!DOCTYPE html>
<html lang="en-US">
<head>
  <title>Just a moment...</title>
  ...
</head>

我已经尝试过以下方案：

• 添加多种User-Agent请求头
• 添加Accept-Encoding和Accept-Language请求头
• 验证所有参数均正确无误（在Postman中可正常工作）
• 检查环境变量已正确配置

内容的提问来源于stack exchange，提问作者OhhhThatVarun