Issue a warning to the user when their HTTP filter is preempted by another user

[Razz4780]

HTTP filtering is a feature of mirrord that allows the user to steal only a subset of HTTP requests coming to the remote target (see reference here and here.

When the mirrord-agent captures a request, it compares it against HTTP filters from all connected clients here. It may happen that the request matches more than one filter. When this happens, only one of the clients receives the request, and the other clients don't even know it happened. We want to notify them about this event to improve the UX. For now, a simple warning should be enough.

Also, we'd like to have a new unit test for this logic (unless it turns out it's already covered by existing FilteredStealTask unit tests).

[linear]

MBE-745 Issue a warning to the user when their HTTP filter is preempted by another user