Prevent stale authentication state across requests in persistent workers

Problem/Motivation

In FrankenPHP worker mode, requests reuse the kernel and services. AuthenticationSubscriber::onKernelRequestAuthenticate()sets the current account only when a provider returns one. If the provider returns nothing or an exception is thrown, the current_user proxy keeps the account from the previous request.

Steps to reproduce

1. Use a test authentication provider that returns an authenticated user.
2. Handle and terminate a main request for the authenticated user.
3. Using the same kernel and container, handle another main request for an anonymous user.
4. Observe that the current user is still the authenticated user. It should be anonymous.

Proposed resolution

At the start of each main request, set the current user to anonymous before checking providers for each main request.

AI disclosure: I used AI to help prepare the IS and MR (Astra and Opus 5.5).