Flakey Test for Sharding upgrade on preflight check

One of the tests for pre-upgrade-check on a sharded cluster is failing. It goes past the timeout waiting for the model to settle.

The test that is failing is testing a network cut on the sharded cluster, waiting for idle, then running the pre-upgrade check. However it is the waiting for idle portion that fails

It was seen failing recently in these two PRs([one](https://github.com/canonical/mongodb-operator/actions/runs/10097625320/job/27948010320?pr=438) and [two](https://github.com/canonical/mongodb-operator/actions/runs/10096231543/job/27918535449?pr=439))

Start with the pre-upgrade-check test for a network cut on a sharded cluster, focusing on the wait-for-idle step that exceeds its timeout. Compare the failures linked from PRs 438 and 439 and trace the test's setup and settling behavior. Done means the test reliably reaches pre-upgrade check without timing out.

Written by the indexing model from the issue text.