Use a single molecule test job (no matrix) on ubuntu-2204 only. This
stops the workflow immediately when the first scenario fails instead of
wasting time running 3 parallel runners that all fail for the same
reason. Multi-platform will be restored once the base scenarios pass.
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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>
