Issue #3619196: Assert the path processor scope without a web server in between

The test that asserts the processor leaves the rest of the site alone failed in full runs and passed on its own, which read as a dependency on the tests before it. It was not one. The failure is a 502, and the web server says why: the test harness turns the cacheability debug headers on, the resulting X-Drupal-Cache-Tags header on an admin page runs past the FastCGI buffer, and the server refuses to pass the response on. This test looked singled out only because it was one of the two that asked for an admin page.

The guarantee itself does not need a request. It is asserted directly now, in a kernel test, over eleven paths of the rest of the site, each with and without the query parameter this module reads, and over language-prefixed variants of them. Both sides of the boundary are covered, so the test cannot pass against a processor that answers for nothing: this module's own endpoint under the prefix of the default language is still refused, and an alias still resolves onto the endpoint that serves the entity behind it.

The browser test keeps what needs a real request and asks for a small page rather than two of the heaviest ones in Drupal, and the README documents the 502 so the next person reads the server log instead of re-running the suite.

Closes #3619196