Rmd handling in Positron is broken when there is a triple backtick immediately followed by prose on the next line

See https://github.com/posit-dev/positron/issues/4155

Create an Rmd with the following in it

Lets say you have two variables `x` and `y` that both point to the same underlying data.

```{r}
x <- c(1, 2, 3)
y <- x
```
If you modify `y`, R will first copy the values of `x` to a new position, then point `y` to the new location and only after the copy modify `y`.

```{r}
x <- 1
```

Note the triple backticks immediately followed by If you on the next line (no newline between them). This seems to cause issues.

Place your cursor on x <- 1 and try to run that line with Cmd + Enter, it will instead send this massive block to the console (in Positron):

`
If you modify `y`, R will first copy the values of `x` to a new position, then point `y` to the new location and only after the copy modify `y`.

```{r}
x <- 1
```

Also notice how the highlighting is off here:

A triple backtick immediately followed by prose on the next line works fine in RStudio (and highlights fine) so I think it should be supported here too.

quarto check

[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.1.2: OK
      Dart Sass version 1.55.0: OK
      Deno version 1.33.2: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.4.185
      Path: /Applications/quarto/bin

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.12.2
      Path: /Users/davis/.pyenv/versions/3.12.2/bin/python3
      Jupyter: 5.7.2
      Kernels: ark, python3

[✓] Checking Jupyter engine render....OK

[✓] Checking R installation...........OK
      Version: 4.4.1
      Path: /Library/Frameworks/R.framework/Versions/4.4-arm64/Resources
      LibPaths:
        - /Users/davis/Library/R/arm64/4.4/library
        - /Library/Frameworks/R.framework/Versions/4.4-arm64/Resources/library
      knitr: 1.48
      rmarkdown: 2.27

[✓] Checking Knitr engine render......OK

Start with the reproduction in this issue and the linked Positron issue #4155, using the Rmd example and the Cmd + Enter entry point. Compare the affected code-block highlighting and executed selection with the expected boundaries when prose follows a triple backtick; done means both highlighting and line execution handle that layout correctly.