Handle keep-ipynb correctly now that fileInformationCache is responsible for the cleaning

[cderv]

This is a first proposal to fix #12780

It is based on the fact that

• Clean up transients from quarto inspect #12337 introduced a global fileInformationCache in the project context, that does the cleaning. For quarto inspect but also for quarto render - without taking into account keep-ipynb

So this PR does now in execute() set transient: false to prevent cleaning when keep-ipynb: true

The cleaning is still done by fileInformationCache cleanup step.

To ensure that happens, the safeCloneDeep escape mode is leveraged to ensure that the project context fileInformationCache will always be accessible by reference.

I said first proposal because there is supposed to be a cleaning mechanism in engine with
executeTargetSkipped and so I don't see why #12337 was requiring this fileInformationCache cleanup.

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