Positron cannot recognize slides with multiple `column` environment correctly

Bug description

Transferred from https://github.com/posit-dev/positron/issues/6802

System details:

Positron and OS details:

Positron Version: 2025.03.0 (user setup) build 116
Code - OSS Version: 1.96.0
Commit: 7f0f93873e8b24c9f393bb3586724f4783c5e72c
Date: 2025-03-01T19:05:24.180Z
Electron: 32.2.6
Chromium: 128.0.6613.186
Node.js: 20.18.1
V8: 12.8.374.38-electron.0
OS: Windows_NT x64 10.0.26100

Interpreter details:

R 4.5.0

Describe the issue:

When adding multiple columns environment, Positron cannot recognize later sections. The code chunk cannot be executed, either.

The following codes create a minimal reproducible example: in which the sections after # C do not shown in the OUTLINE, but everything shows well in Rstudio. See the comparison of the screenshots below:

Steps to reproduce the issue:

---
title: "Untitled"
format: html
---

# A

## B

:::: {.columns}

::: {.column width="40%"}
left

::: {.fragment}
body
:::
:::

::: {.column width="30%"}
right
:::

::: {.column width="30%"}
right
:::


::::

## B

:::: {.columns}

::: {.column width="40%"}
left

::: {.fragment}
body
:::
:::

::: {.column width="30%"}
right
:::

::: {.column width="30%"}
right
:::


::::

## B

:::: {.columns}

::: {.column width="40%"}
left

::: {.fragment}
body
:::
:::

::: {.column width="30%"}
right
:::

::: {.column width="30%"}
right
:::


::::

## B

:::: {.columns}

::: {.column width="40%"}
left

::: {.fragment}
body
:::
:::

::: {.column width="30%"}
right
:::

::: {.column width="30%"}
right
:::


::::

## B

:::: {.columns}

::: {.column width="40%"}
left

::: {.fragment}
body
:::
:::

::: {.column width="30%"}
right
:::

::: {.column width="30%"}
right
:::

::::

# C

## D

```{r}
"hello world"
```

Expected or desired behavior:

Make positron show the outline correctly and make the chunks executable.

Steps to reproduce

No response

Actual behavior

No response

Expected behavior

No response

Your environment

No response

Start with the supplied minimal Quarto Markdown example, focusing on how repeated .columns and .column blocks affect the outline and R chunk execution. Trace the outline and chunk-handling entry points, then verify that sections after # C appear in the outline and that the final R chunk is executable.