YAML Validation fails for specifying filter process positions in extensions outside of formats

Bug description

From More precise targeting of AST processing phases in Quarto v1.4 pre-release, there are different injection points for Lua filters. I'm in need of a filter that intercepts the format or main meta data in advance of Quarto creating the website/html envelope so that I can sneak links into the right hand side menu bar.

However, I'm running into an error with respect to packaging the new targets, specifically we have:

title: Astdemo
author: JJB
version: 1.0.0
quarto-required: ">=1.6.0"
contributes:
  filters:
    - at: pre-ast
      path: astdemo.lua

Causing on quarto preview:

ERROR: YAML Validation Failed

In file _extensions/astdemo/_extension.yml
(line 7, column 7 through line 8, column 23) Array entry 1 with value at: pre-ast
  path: astdemo.lua failed to be a string.
6:   filters:
7:     - at: pre-ast
        ~~~~~~~~~~~~
8:       path: astdemo.lua
  ~~~~~~~~~~~~~~~~~~~~~~~
✖ The value

at: pre-ast
  path: astdemo.lua

is of type object.
ℹ The error happened in location contributes:filters:0.

A quick grep on existing code extensions just turns up apaquarto, which has this specification under contributes.formats.common and subsequently under more specific format outputs, e.g. contributes.formats.html et cetera:

title: My Document in APA Style, Seventh Edition
author: W. Joel Schneider
version: 4.3.6
quarto-required: ">=1.4.549"
contributes:
  formats:
    common:
      execute:
        echo: false
      citeproc: false
      link-citations: true
      # .... suppressed more
      filters:
        - at: pre-ast
          path: apalanguage.lua
        - at: pre-ast
      # .... suppressed more

Steps to reproduce

Please see the extension code in https://github.com/coatless-quarto/filter-processing

This is just the default template with the above _extension.yml modification.

Expected behavior

I'd expect to be able to use the filter outside of the format.

Actual behavior

The filter must be placed under a new format.

Your environment

• IDE: VS Code 1.95.3
• OS: macOS 15.1.1 (24B91)

Quarto check output

Quarto 1.6.39
[✓] Checking environment information...
      Quarto cache location: /Users/ronin/Library/Caches/quarto
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.4.0: OK
      Dart Sass version 1.70.0: OK
      Deno version 1.46.3: OK
      Typst version 0.11.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.6.39
      Path: /Applications/quarto/bin

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

[✓] Checking LaTeX....................OK
      Using: Installation From Path
      Path: /opt/homebrew/bin
      Version: undefined

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

[✓] Checking Python 3 installation....OK
      Version: 3.11.10
      Path: /Users/ronin/.pyenv/versions/3.11.10/bin/python
      Jupyter: (None)

      Jupyter is not available in this Python installation.
      Install with python3 -m pip install jupyter

[✓] Checking R installation...........OK
      Version: 4.4.2
      Path: /Library/Frameworks/R.framework/Resources
      LibPaths:
        - /Library/Frameworks/R.framework/Versions/4.4-arm64/Resources/library
      knitr: 1.49
      rmarkdown: 2.29

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