Mixing Julia cells with Python cells breaks `echo` metadata

[denius]

When using Julia code cells with Python cells, attempting to specify echo metadata results in a YAMLException.

For demonstration, I took the file https://github.com/PumasAI/QuartoNotebookRunner.jl/blob/main/test/examples/integrations/PythonCall.qmd included in the project, shortened it a bit and added a couple of echo options:

---
title: PythonCall integration
julia:
    exeflags: ["--project=PythonCall"]
---

```{julia}
#| echo: false
using PythonCall
```

```{python}
#| echo: fenced
import re
```

The following error occurs when executing the rendering command quarto render PythonCall.jl.qmd --to html in Quarto 1.7.31:

Starting julia control server process. This might take a while...
Julia server process started.
Running [1/2] at line 7:  using PythonCall
Running [2/2] at line 12:  import re
ERROR: YAMLException: duplicated mapping key (undefined, 3:1)
3: echo: fenced
  ~~

When using only Julia code cells, such errors do not occur.

[jkrumbiegel]

Ah yes behind the scenes an invisible julia cell is created that actually runs your code, and this one sets echo: false to be invisible, but currently other echo settings are not removed so you get this validation error. You could check the code that sets up the fake cell and remove possible echo lines from the code there