### Bug description

Using Quarto 1.3.340 on macOS.

Given the following Markdown file named `index.md`, Quarto reports that the file must be named `*.qmd` because it contains executable code. There is no executable code in this document.

`````markdown
# Simple Markdown

This is a simple Markdown document containing an embedded R Markdown document.

````markdown
---
title: "embedded"
---

This is an embedded R Markdown document contained within a simple Markdown
document.

```{r setup, echo = FALSE, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```
````
`````

The resulting error:

```bash
quarto render index.md
#> ERROR: You must use the .qmd extension for documents with executable code.
#> 
#> Error: You must use the .qmd extension for documents with executable code.
#>     at Object.execute (file:///Applications/RStudio.app/Contents/Resources/app/quarto/bin/quarto.js:55533:23)
#>     at renderExecute (file:///Applications/RStudio.app/Contents/Resources/app/quarto/bin/quarto.js:86465:48)
#>     at renderFiles (file:///Applications/RStudio.app/Contents/Resources/app/quarto/bin/quarto.js:86588:57)
#>     at async render (file:///Applications/RStudio.app/Contents/Resources/app/quarto/bin/quarto.js:90675:21)
#>     at async Command.fn (file:///Applications/RStudio.app/Contents/Resources/app/quarto/bin/quarto.js:90850:32)
#>     at async Command.execute (file:///Applications/RStudio.app/Contents/Resources/app/quarto/bin/quarto.js:8437:13)
#>     at async quarto (file:///Applications/RStudio.app/Contents/Resources/app/quarto/bin/quarto.js:127535:5)
#>     at async file:///Applications/RStudio.app/Contents/Resources/app/quarto/bin/quarto.js:127553:9
```

Encountered when trying to adapt a mkdocs site to use Quarto without performing a bunch of file renaming.

### Checklist

- [x] Please include a minimal, fully reproducible example in a single .qmd file? Please provide the whole file rather than the snippet you believe is causing the issue.
- [x] Please [format your issue](https://quarto.org/bug-reports.html#formatting-make-githubs-markdown-work-for-us) so it is easier for us to read the bug report.
- [x] Please document the RStudio IDE version you're running (if applicable), by providing the value displayed in the "About RStudio" main menu dialog?
- [x] Please document the operating system you're running. If on Linux, please provide the specific distribution.
- [x] Please provide the output of `quarto check` so we know which version of quarto and its dependencies you're running.
cscheid
2023-05-18T18:44:02Z
aronatkins
bug ast
Future
open
document without executable code reported as containing executable code
https://github.com/quarto-dev/quarto-cli/issues/5602