fix: override exported content nodes duration to completion criteria threshold

[vkWeb]

Summary

When a node's completion criteria has a model of either time or approx_time, the exported ContentNode created during channel publishing is set to have the threshold written to the duration field.

Reviewer guidance

Does this PR performs the expected overriding behaviour?

References

Closes #3322.

Contributor's Checklist

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[bjester]

Small tweaks, looking good

[bjester]

Looks like the python tests stalled again. Rebasing after #3334 is merged should hopefully help