Summary

Fixes #43515.

On Windows, the ffmpeg process that Playwright starts for video recording gets a visible console window of its own. launchProcess() never passed windowsHide to child_process.spawn(), so ffmpeg inherits (or creates) a console - since #42155 the CLI session daemon runs detached with windowsHide: true and has no console of its own, any console program it starts (ffmpeg) gets a brand new window, which steals focus every time a recording starts.

#41630 asked for windowsHide: true on every launchProcess call, and was closed as not planned: an earlier attempt at exactly that (#38776) was reverted in #39994 after it broke Inspector, codegen and UI mode (#39990) - those child processes have real windows of their own that windowsHide would also hide.

This PR makes windowsHide an opt-in field on LaunchProcessOptions (default unset, so behavior elsewhere is unchanged) and sets it only for the ffmpeg launch in videoRecorder.ts, since ffmpeg has no GUI and nothing to hide incorrectly.

// packages/utils/processLauncher.ts
const spawnOptions: childProcess.SpawnOptions = {
  detached: process.platform !== 'win32',
  env: options.env,
  cwd: options.cwd,
  shell: options.shell,
  windowsHide: options.windowsHide,   // new, opt-in
  stdio,
};

// packages/playwright-core/src/server/videoRecorder.ts
const { launchedProcess, gracefullyClose } = await launchProcess({
  command: this._ffmpegPath,
  args,
  stdio: 'stdin',
  windowsHide: true,                  // new
  ...

Test plan

• The reporter validated this exact change on real Windows 11 hardware: polled every visible top-level window every 250ms during a recording with the patch applied, confirmed no window appeared, and confirmed the recorded webm was still written correctly.
• windowsHide is a Windows-only child_process option (no-op elsewhere per Node docs), so there's nothing to assert about window visibility in CI on Linux/macOS, and the prior related fix for the same file (fix(windows): hide console window when spawning browser processes #38776) shipped the same way, with manual Windows verification only, no automated test.
• Ran the full video.spec.ts suite (31 tests) on macOS with the change - all pass, confirming the new field doesn't change ffmpeg's behavior off Windows.
• npx eslint and npx tsc -p . on the changed files - clean.

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