R version in Terminal doesn't match R version in Console (`PATH`, `QUARTO_R`, etc.)

[jmcphers]

System details:

Positron and OS details:

Positron Version: 2025.05.0 (Universal) build 86
Code - OSS Version: 1.99.0
Commit: 64988dfaeaa5de9219cb777569c2e8631aefbe0e
Date: 2025-04-25T13:51:33.661Z
Electron: 34.3.2
Chromium: 132.0.6834.210
Node.js: 20.18.3
V8: 13.2.152.41-electron.0
OS: Darwin arm64 24.4.0

Interpreter details:

Any

Describe the issue:

The version of R used in the Terminal does not match the one used in Positron's console.

Steps to reproduce the issue:

Use a system with multiple versions of R installed, then:

1. Start a version of R that is not on the PATH (that is, it is not the R you get when running R in the Terminal)
2. Open a Terminal and check the R version there.

It is not the version of R you selected in Positron, but your system default version of R.

Expected or desired behavior:

The version of R in the Terminal should match the version of R in the Console, for a few reasons:

• Several existing extensions (e.g Quarto Preview, Shiny Run App) start R in the Terminal to perform background tasks. Those tasks can fail or produce unexpected results if they don't run against the R installation users expect.
• The Python extension (as seen in the screenshot above) does customize the Terminal to point at the user selected version of Python. The R extension should do the same for consistency.

Originally noted by @cwickham.

[juliasilge]

One thing I want to highlight from what I wrote up in #7402 is that the terminals don't need to update something like PATH or QUARTO_R but rather just have the right value when they launch. This is what the Python extension does and it works well, including for Quarto.

• Start one Python console with Python version A
• Launch a terminal; it uses Python version A
• Start an additional Python console with Python version B
• Launch an additional terminal; this one now uses Python version B
• The first terminal is still using Python version A

[jmcphers]

RSTUDIO_PANDOC would be good to set in terminals, too (cf. #7700)

[jmcphers]

Might be the cause of https://stackoverflow.com/questions/79711192/quarto-cant-find-packages-installed-with-renv-positron