[6.x] Stop the Playwright server at the end of browser test runs - #20452

Description

pest-plugin-browser 5.2.0 starts the Playwright server with sh -c and no exec. On Linux, dash forks node as a child, so stop() at the end of the run kills the shell and node keeps running:

 php vendor/bin/pest
-└─ sh -c ./node_modules/.bin/playwright run-server …   ← stop() signals this
-   └─ node … run-server                                 ← orphaned, holds Pest's output pipe
+└─ node … run-server                                   ← exec: stop() signals the server itself

Every browser run leaves a server behind, and because node keeps a copy of Pest's output pipe, vendor/bin/pest tests/Browser/… | tail never returns.

tests/Pest.php now creates the plugin's PlaywrightNpmServer with exec in front of the command and stores it in ServerManager, which builds its server with ??= and keeps it. It only applies on non-Windows systems with the plugin at 5.2.0 or earlier, because a fixed release adds its own exec and exec exec fails. Remove it once the upstream fix is released.

Evidence

• Before: pest tests/Browser/Cp/AuthenticationTest.php | tail -3 hangs (over 2.5 min); an orphaned node … run-server with parent PID 1 holds tail's input pipe. pest --parallel --processes=2 tests/Browser/Cp/Settings: 16 passed, one run-server left running.
  After: the piped run returns in about 15s, 2 of 2 runs, no processes left. The parallel run: 16 passed, no processes left. Unit tests start no server.

Related issues

• fix: stop the playwright server process, not only its shell pestphp/pest-plugin-browser#281
• Stop the Playwright server process, not its shell pestphp/pest-plugin-browser#304