Validate cached venv by testing pytest, not just python binary

The cached venv's python binary may work but tool symlinks (pytest, etc.)
break when restored in a different environment. Test pytest specifically
to catch this.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/deploy.yml

@@ -55,7 +55,7 @@ jobs:
           cd dashboard/backend
           if [ "${{ steps.cache-python.outputs.cache-hit }}" = "true" ]; then
             echo "Restoring venv from cache $CACHE_KEY..."
-            if cp -a $CACHE_DIR/venv . && [ -f venv/bin/activate ] && venv/bin/python -c "import sys; sys.exit(0)"; then
+            if cp -a $CACHE_DIR/venv . && [ -f venv/bin/activate ] && venv/bin/python -m pytest --version >/dev/null 2>&1; then
               echo "Cache restored successfully"
             else
               echo "Cache corrupted, installing fresh..."