Still generating diagnostics for `.vdoc` files when a formatter is set up

[juliasilge]

Related to #832 where we did work to turn off diagnostics in the virtual documents

If you have a Quarto file with a syntax error such as this:

---
title: "Untitled"
format: html
---

Hi!

```{python}
x = {"x1": 1, "x2": 2 "x3": 3, "x4": 4}
```

Bye!

And you have a formatter installed and turned on for the underlying language, like this in settings.json:

    "[python]": {
        "editor.defaultFormatter": "charliermarsh.ruff"
    },

Then you can still get a PROBLEM reported for the underlying virtual document:

This problem also occurs in Positron, and applies to both R and Python if you have a formatter for either installed.

It's a little tricky to get the diagnostics to show up, but I typically can get it to happen if I edit a character or two in the code cell. We have a report in posit-dev/positron#4482 (reply in thread) of this happening when you format the document but I did not find that to always cause this to happen.