Respect quarto.cells.useReticulate setting in Positron

[juliasilge]

Addresses posit-dev/positron#6675 in part

This PR fixes a bug in Positron. Before this change, Positron ignored the quarto.cells.useReticulate setting. When you ran a Python cell in a document with engine: knitr, Positron always rewrote the code to run in the R console through reticulate::repl_python(...). The setting had no effect. Now the Positron cell execution path checks the same setting that the default VS Code host already uses.

I also added a regression test. The test runs a knitr Python cell with the setting off. It shows that the cell executes in the Python console instead of R.

Known limitation

This change does not solve the full problem in the issue about the difference between running a statement and Run Cell. In Positron, Cmd+Enter (run the current statement) sends the command to Positron core's positron.executeCodeFromPosition command. This command always runs Python cells on the Python console. It does not check the quarto.cells.useReticulate setting, and this extension has no hook to change that path. As a result, with the setting on, Run Cell now goes to R through reticulate, but Run Line still goes to Python. A fix for that part of the issue needs a change in Positron itself.

Validation steps

• yarn test-positron passes (includes the new test)

• With a .qmd file that has both an r chunk and a python chunk:

  • With the setting on (the default), running a knitr Python cell with Cmd+Shift+Enter runs it via R through reticulate:

• With the setting off, the same cell runs directly in the Python console:

• R cells run the same way, with the setting on or off

• Switching the setting on and off again repeats this same behavior each time

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[vezwork]

The code change is simple and good. Tested in Positron daily confirmed that by default (when the useReticulate setting is on) python cells in a qmd with an r cell get sent to the r console with reticulate. When the setting is off, they get sent to the python console.

Looking forward to command+enter being fixed to respect the setting as well.