[FrameworkBundle] Add a "configure_container" option to WebTestCase::createClient() - #65997

KernelBrowser::doRequest() reboots the kernel on every request after the first one. That includes crawler-driven requests: click(), submit() and redirect following all go through the same code path. So a service replaced with self::getContainer()->set(...) survives exactly one request and then silently vanishes: the test gets the real service back, with no error and no warning.

This adds a configure_container option to WebTestCase::createClient(), holding a closure that is applied to the container after every kernel boot, for the lifetime of the client:

$client = self::createClient(['configure_container' => function (ContainerInterface $container) {
    $container->set(ClockInterface::class, $clock);
}]);

$crawler = $client->request('GET', '/');
$clock->sleep(3600);
$client->click($crawler->selectLink('Next')->link()); // reboots, and the closure is applied again

The closure receives the container returned by KernelBrowser::getContainer(), which resolves test.service_container when available, so private services can be replaced. It runs on a freshly booted, untouched container, which is required since TestContainer::set() refuses to replace a private service that has already been initialized.

Why $options and not a new parameter on createClient(): overriding createClient() in a project base test case is a very common pattern, and PHP fatals when a child method declares fewer parameters than its parent. Adding a third argument would break every such override. KernelTestCase::createKernel() already documents $options as an extensible bag and reads only the keys it knows about, so this fits. createClient() extracts the key and unset()s it before booting, so a user-defined createKernel() never sees the closure.

Notes:

• with disableReboot() there is no new boot, so the closure is not re-applied (the container simply keeps what was set);
• insulated requests run in a separate process and serialize their state, which a closure cannot cross, so registering a configurator and calling insulate() now throws a clear LogicException instead of failing obscurely;
• passing a non-closure under the key throws an InvalidArgumentException;
• KernelBrowser::setContainerConfigurator() is public but @internal for now, to keep the public surface minimal. It can be opened up later if re-registering mid-test turns out to be useful.

The documentation is symfony/symfony-docs#22979. Note that testing.rst currently recommends $container->set(...) under "Mocking Dependencies" with no mention of the reboot, so that PR also adds a warning there.

Thanks to @lyrixx for reporting the issue, and to @nikophil and @benito103e who both volunteered to implement it and never got an answer.