fix: CVE-2026-25990 - update Pillow to >=12.1.1

[VedantMahabaleshwarkar]

• Add Pillow >=12.1.1 constraint to kserve/pyproject.toml (Out-of-bounds Write via Specially Crafted PSD Image)
• Update Pillow constraint in custom_model, custom_transformer, and artexplainer to match
• Regenerate poetry.lock files

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes https://issues.redhat.com/browse/RHOAIENG-44976, https://issues.redhat.com/browse/RHOAIENG-49465

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• Have you linked the JIRA issue(s) to this PR?

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