DefaultExceptionHtmlSubscriber leaves the router with a detached GET request context after error-page subrequests

Problem/Motivation

DefaultExceptionHtmlSubscriber::makeSubrequest() gives router.no_access_checks a cloned request context with its method set to GET, so it can route to the error page. It never restores the original context afterwards. This means that POST is matched to a stale GET.

This affects anything that handles more than one request with one container, such as kernel tests and persistent application servers like FrankenPHP worker mode.

PathValidator::getAttributes() already restores the router context after it changes it.

Steps to reproduce

This behavior appears when one container handles multiple requests (for example, FrankenPHP in worker mode, or a script that calls http_kernel repeatedly).

1. Install Drupal with the Standard profile to get the basic_html editor and POST-only route ckeditor5.upload_image.
2. Save this as repro.php and run with drush php:script repro.php:
   

   <?php
   use Symfony\Component\HttpFoundation\Request;
   
   $kernel = \Drupal::service('http_kernel');
   foreach ([1, 2] as $i) {
     $request = Request::create('/ckeditor5/upload-image/basic_html', 'POST');
     $response = $kernel->handle($request);
     echo "Request $i: " . $response->getStatusCode() . PHP_EOL;
   }
   $context_is_shared = \Drupal::service('router.no_access_checks')->getContext()
     === \Drupal::service('router.request_context');
   echo 'Router uses shared context: ' . var_export($context_is_shared, TRUE) . PHP_EOL;
   

Expected: Both requests return 403, because access is denied. The router still uses the shared router.request_context service.

Actual:

Request 1: 403
Request 2: 405
Router uses shared context: false

The 403 in request 1 is rendered by DefaultExceptionHtmlSubscriber::makeSubrequest(). That method gives the router a cloned context with its method set to GET and never restores the original. RouterListener keeps updating the shared router.request_context for each new request, but the router now holds the detached clone. Router::doMatchCollection() checks $this->context->getMethod(), so request 2 is matched as a GET, fails the methods: [POST] requirement, and returns 405 Method Not Allowed. The router also keeps checking the host against the detached context from request 1.

Proposed resolution

Save the router's context before the subrequest and restore it in a finally block. This way, it is restored even if route matching or the subrequest throws.