disallow `format: pdf: pdf-engine: typst` and do not advertise the option in YAML intelligence

I have:

• searched the issue tracker for similar issues
• installed the latest version of Quarto CLI
• formatted my issue following the Bug Reports guide

Bug description

I get an error when trying to convert a document to a pdf with the typst pdf-engine if the document contains a callout. Based on the documentation I already assumed that I might not get a nicely formatted callout and potentially only a block quote or something, but an actual error was unexpected.

Steps to reproduce

---
author: "Test User"
date: "2025-08-31"
format:
  pdf:
    pdf-engine: typst
---

# Callout test in typst

::: {.callout-tip title="Some callout"}
not actually a tip
:::

Actual behavior

$ quarto render testq.qmd

pandoc 
  to: pdf
  output-file: testq.pdf
  standalone: true
  pdf-engine: typst
  variables:
    graphics: true
    tables: true
  default-image-extension: pdf
  
metadata
  block-headings: true
  author: Test User
  date: '2025-08-31'
  
error: unknown variable: callout
    ┌─ toPdfViaTempFile54304-0.typ:132:1
    │
132 │ #callout(
    │  ^^^^^^^

Error producing PDF.

Expected behavior

Producing a PDF. Maybe not pretty, but something valid.

Your environment

• Sublime Text
• MacOS 15.6.1

Quarto check output

Quarto 1.7.34
[✓] Checking environment information...
      Quarto cache location: /Users/franzi/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: 1.7.34
      Path: /Users/franzi/Applications/quarto/bin

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

[✓] Checking LaTeX....................OK
      Using: Installation From Path
      Path: /Library/TeX/texbin
      Version: 2025

[✓] Checking Chrome Headless....................OK
      Using: Chrome found on system
      Path: /Applications/Google Chrome.app/Contents/MacOS/Google Chrome
      Source: MacOS known location

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

[✓] Checking Python 3 installation....OK
      Version: 3.12.9
      Path: /Users/franzi/.venv/bin/python3
      Jupyter: 5.7.2
      Kernels: python3

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

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

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