`expandable` option not accessible in tabs / tabsets

Bug description

In dashboard, the tabsets makes each tab behave as a card, thus by default add the "expand" feature to it.
Unfortunately, the option cannot be disable at the tab or tabset level contrary to cards and code-cell.

Steps to reproduce

Input	Output
--- title: "Untitled" format: dashboard --- ## Quarto {.tabset} ### Tab1 {{< placeholder 600 400 >}}

Actual behavior

The following has no effects:

---
title: "Untitled"
format: dashboard
---

## Quarto {.tabset expandable="false"}

### Tab1

{{< placeholder 600 400 >}}

---
title: "Untitled"
format: dashboard
---

## Quarto {.tabset}

### Tab1 {expandable="false"}

{{< placeholder 600 400 >}}

Expected behavior

No response

Your environment

No response

Quarto check output

Quarto 99.9.9
[✓] Checking environment information...
      Quarto cache location: /Users/mcanouil/Library/Caches/quarto
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.6.3: OK
      Dart Sass version 1.85.1: OK
      Deno version 1.46.3: OK
      Typst version 0.13.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 99.9.9
      commit: 79a1b18574a3bc8b3d85c85f980edd2dde9fe195
      Path: /Users/mcanouil/Projects/quarto-dev/quarto-cli/package/dist/bin

[✓] Checking tools....................OK
      TinyTeX: v2025.03.10
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /Users/mcanouil/Library/TinyTeX/bin/universal-darwin
      Version: 2025

[✓] Checking Chrome Headless....................OK
      Using: Chrome found on system
      Path: /Applications/Brave Browser.app/Contents/MacOS/Brave Browser
      Source: QUARTO_CHROMIUM

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

[✓] Checking Python 3 installation....OK
      Version: 3.13.1
      Path: /Users/mcanouil/Projects/quarto-dev/quarto-playground/.venv/bin/python3
      Jupyter: 5.7.2
      Kernels: julia-1.11, python3

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

(\) Checking R installation...........ℹ R version 4.5.0 (2025-04-11)
! Config '~/.Rprofile' was loaded!
[✓] Checking R installation...........OK
      Version: 4.5.0
      Path: /Library/Frameworks/R.framework/Resources
      LibPaths:
        - /Users/mcanouil/Projects/quarto-dev/quarto-playground/renv/library/macos/R-4.5/aarch64-apple-darwin20
        - /Users/mcanouil/Library/Caches/org.R-project.R/R/renv/sandbox/macos/R-4.5/aarch64-apple-darwin20/4cd76b74
      knitr: 1.50
      rmarkdown: 2.29

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

Start by rendering the provided dashboard .qmd examples and compare tabset-level and tab-level expandable behavior. Trace the dashboard tabset handling from that reproduction; done means expandable="false" disables expansion at both supported levels.