document without executable code reported as containing executable code

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.

# 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:

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

• 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.
• Please format your issue so it is easier for us to read the bug report.
• Please document the RStudio IDE version you're running (if applicable), by providing the value displayed in the "About RStudio" main menu dialog?
• Please document the operating system you're running. If on Linux, please provide the specific distribution.
• Please provide the output of quarto check so we know which version of quarto and its dependencies you're running.