Add GitLab CI and a kernel test that signs real files

Part of #3596034 (closed).

Problem/Motivation

The project has no .gitlab-ci.yml. Signing is only tested by hand, so a change in the manifest or a new c2patool release can break it unnoticed.

Proposed resolution

• Add the standard Drupal CI template, with the previous major.
• Install c2patool in the PHPUnit job for the oldest and newest supported versions, 0.27.x and 0.28.x.
• Add a kernel test that signs a plain JPEG and a C2PA 2.x JPEG through the module, then reads them back with c2patool. It skips when c2patool is missing.

Definition of done

• Pipeline runs on every MR and on 1.x
• PHPUnit runs once per c2patool version, and the job log shows c2patool --version
• Plain JPEG: one valid manifest, claim_generator_info with name and icon
• C2PA 2.x JPEG: two manifests, the parent ingredient keeps the original one
• Without c2patool the test is skipped, not failed
• Linters do not block the pipeline until their existing errors are fixed

Testing instructions

1. Open the MR pipeline. Both c2patool jobs are green on the current and previous major.
2. In a test branch, require jrglasgow/c2patool 0.5.2. The icon case fails.
3. Run the test locally without c2patool. It reports 2 skipped.

AI Compliance

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