Add a kernel test that signs an image style derivative

Part of #3596034 (closed).

Problem/Motivation

SignImageTest signs uploaded originals only. Signing of image style derivatives, in C2paSignSubscriber with the drupal.c2pa_sign.image_derivative assertion, has no test. Modules such as ai_disclosure_image add their actions on that path, so a regression there would pass CI.

Proposed resolution

Add one kernel test, with no runtime change. It builds a style with Scale and Convert, requests a derivative of a signed original with derivative signing on, and reads the result with c2patool.

Definition of done

• The derivative has a valid manifest with a parentOf ingredient
• The actions match the style effects
• The drupal.c2pa_sign.image_derivative assertion is present
• The test runs in CI on c2patool 0.27.22 and 0.28.2

AI Compliance

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