[Regression]: Request body is not correct when copying request

[cpAdm]

Last Good Version

1.56

First Bad Version

1.57

Steps to reproduce

1. Open UI mode
2. Run following test

import { test, request, expect } from '@playwright/test';


test('fetch call to example site with payload', async ({}) => {
    const ctx = await request.newContext();
    const res = await ctx.post('https://httpbin.org/post', {
        data: {message: 'Hello from Playwright!', number: 123},
        headers: {'Content-Type': 'application/json'},
    });
    await expect(res).toBeOK();
    await ctx.dispose();
});

3. Open network tab
4. Select post request
5. Click copy request (e.g. as fetch)

Expected behavior

Expected body:
"body": "{\"message\":\"Hello from Playwright!\",\"number\":123}",

Actual behavior

Body:
"body": "{\"error\":\"trace parameter is missing\"}",

Additional context

(also tested with changes from #38566, still an issue)

Environment

System:
    OS: Linux 6.6 Ubuntu 24.04.2 LTS 24.04.2 LTS (Noble Numbat)
    CPU: (12) x64 Intel(R) Core(TM) i5-10505 CPU @ 3.20GHz
    Memory: 5.41 GB / 19.53 GB
    Container: Yes
  Binaries:
    Node: 22.15.1 - /usr/bin/node
    Yarn: 1.22.22 - /usr/bin/yarn
    npm: 10.9.2 - /usr/bin/npm
  Languages:
    Bash: 5.2.21 - /usr/bin/bash

[pavelfeldman]

@cpAdm do you want to take a look?