Undefined variable $target in C2paSignSubscriber

Part of #3596034 (closed).

Problem/motivation

C2paSignSubscriber::onKernelResponse() reads $request->query->get('file') ?? $target. $target is only set when the route match is empty, so an image style request without a file query parameter logs Undefined variable $target.

Proposed resolution

Initialize $target to NULL before the route check.

AI Compliance

🤖 Generated with the help of an LLM, reviewed and approved by me before posting.