Manage projectcontext objects correctly on preview

[cscheid]

This should close a few of our quarto preview intermittent issues whose symptom is a missing quarto_ipynb file.

See, eg, #12992.

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[cscheid]

This last failing test is interesting. If I'm reading the messages correctly, we previously had:

• a test that used to pass but which requested invalid options
• a code path that verified the validity of options that wasn't getting called because of incorrect plumbing of the ProjectContext object

Now that things are properly connected, the test fails. But it seems that this test shouldn't really even be there?