Function planning_session_context_with_join_threshold 

pub fn planning_session_context_with_join_threshold(
    target_partitions: usize,
    spill_join_build_bytes: Option<u64>,
) -> SessionContext

As planning_session_context, with the spillable-join build-side threshold supplied instead of derived from this process’s cgroup.

None keeps the derived threshold, which is what production uses. Tests pin it because the rule’s behaviour is the whole difference between a 3-core executor with a ~700 MB per-task share and a build box with tens of gigabytes: a defect that only appears once joins actually convert is invisible on the machine the tests run on.