fix(ci): recreate venv after cache restore to fix absolute symlinks
Running python3 -m venv venv over an existing cached venv recreates the absolute symlinks and pyvenv.cfg paths so Python resolves to the current system's python3 binary. Without this, cp -a preserves stale symlinks from the cache source, causing venv/bin/python to resolve to /usr/bin/python3 (system, not venv) and pytest not found. Also switch validation to use python3 to match Ubuntu 24.04 convention.
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