Fix regression around OnIdle handler

[andyleejordan]

Unfortunately the (admittedly hairy) change made in #1918 to fix:

System.Management.Automation.PSInvalidOperationException: The pipeline was not run because a pipeline is already running. Pipelines cannot be run concurrently.

has resulted in PowerShell/vscode-powershell#4219:

System.Management.Automation.PSInvalidOperationException: You should only run a nested pipeline from within a running pipeline.

because we had hacked in that all our pipelines are nested (to allow the concurrency check to pass), but when loading profiles, we do not actually have a running pipeline, so our lie about being nested fails another check.

So I think what happened was that PowerShell/vscode-powershell#4048 was already no longer reproducing by happenstance (as the Az.Tools.Predictor module presumably updated and removed their OnIdle handler). I failed to check that before attempting to fix it following the last result of our investigation. The "fix" inadvertently caused nearly the same PSInvalidOperationException, and by mistake I confused the two scenarios. With this PR, and regression tests, I can confirm that the OnIdle handler works both in a profile and at the regular prompt after reverting the breaking change. Furthermore, I can no longer repro the original issue with Az.Tools.Predictor whatsoever, in any version, hence my conclusion their module updated.

Resolves PowerShell/vscode-powershell#4219

[SeeminglyScience]

LGTM!