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Replace actions/setup-python (which fails on a cold runner toolcache with a broken python-versions prebuilt) with the self-contained uv installer, matching release.yaml. Uses `uv sync --all-extras --locked --python 3.12` and `uv run --locked pytest`, so the test gate — and therefore the versioned image build behind it — no longer depends on a warm toolcache. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NGzHtDvJur9U7ysgRKRUTN