include shortcode ignores project absolute path ("/") when called from a file inside a "_folder".

Bug description

Hi,

Steps to reproduce

My folder structure:
_lessons
_exercises
articles

In "articles" I have several includes like:
{{< include "/_lessons/lesson1.qmd" >}}
{{< include "/_exercise/exercise1.qmd" >}}

This works like a charm. Include files are perfectly found.

BUT...

Problem: include does not work when I call them from a file inside "_lessons".
Example: in file _lessons/lesson2.qmd:
{{< include "/_exercise/exercise1.qmd" >}}

Actual behavior

The compiler/interpreter understand that the path is "_lessons/_exercise/exercise1.qmd" although I use "/" sign in the path. Therefore the file is not found.

This only happens in includes called inside files inside _folders.
Something related to the _folders makes this root path crashes and be ignored.

Expected behavior

I expect that the compiler understand that "/" inside include shortcode (wherever it is, even in "_folders") is understood as the project root.

Your environment

OS: Ubuntu 24.04
VsCode: 1.116.0
Quarto v: 1.9.37

Quarto 1.9.37
[✓] Checking environment information...
Quarto cache location: /home/chema/.cache/quarto
[✓] Checking versions of quarto binary dependencies...
Pandoc version 3.8.3: OK
Dart Sass version 1.87.0: OK
Deno version 2.4.5: OK
Typst version 0.14.2: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
Version: 1.9.37
Path: /opt/quarto/bin

[✓] Checking tools....................OK
TinyTeX: (not installed)
Chromium: (not installed)
Chrome Headless Shell: (not installed)
VeraPDF: (not installed)

[✓] Checking LaTeX....................OK
Using: Installation From Path
Path: /usr/bin
Version: 2023

[✓] Checking Chrome Headless....................OK
Using: Chrome found on system
Path: /usr/bin/google-chrome
Source: PATH

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

[✓] Checking R installation...........(None)

  Unable to locate an installed version of R.
  Install R from https://cloud.r-project.org/

[✓] Checking Python 3 installation....OK
Version: 3.12.3
Path: /home/chema/WORKSPACE/quarto/web_phy-che_privada/.venv/bin/python
Jupyter: 5.9.1
Kernels: python3, web_phy-che_privada

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

[✓] Checking Julia installation...

Start by reproducing the reported folder structure and include calls, then trace how the include shortcode resolves paths when invoked from a file inside a _folder. Done means a leading slash consistently resolves from the project root, including from nested files, and the existing reproduction succeeds.