(gatsby-plugin-mdx): GatsbyConfig typescript is not honoured when options are set

[jcxldn]

Preliminary Checks

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• This issue is not a question, feature request, RFC, or anything other than a bug report directly related to Gatsby. Please post those things in GitHub Discussions: https://github.com/gatsbyjs/gatsby/discussions

Description

When using gatsby-plugin-mdx with gatsby-config.ts, if configuration options are set (such as mdxOptions), gatsby-config data is ignored. In my reproduction, the site title shows as null, and allFile has zero entries (should have one).

If gatsby-plugin-mdx is added without configuration options, everything works fine.

Reproduction Link

https://github.com/jcxldn/gatsby-mdx-reproduction

Steps to Reproduce

In minimal reproduction:

1. Configure gatsby-plugin-mdx with options:

   {
      resolve: `gatsby-plugin-mdx`,
      options: {
        mdxOptions: {
          remarkPlugins: [
            remarkToc
          ]
        }
      }
    }

2. Run gatsby develop --verbose
3. Open GraphiQL and run the following query:

query MyQuery {
  allFile {
    totalCount
  }
  site(siteMetadata: {}) {
    siteMetadata {
      title
    }
  }
}

4. Note that site title is null and totalCount is zero.
5. Configure gatsby-plugin-mdx without options:

{ resolve: `gatsby-plugin-mdx` }

6. Restart the server
7. Note that site title is not null and totalCount is one.

Expected Result

Gatsby's config continues to be honoured and options set in it still apply (such as siteMetadata.title)

Actual Result

Config is not honored when options are set.

Environment

System:
    OS: Linux 6.4 Manjaro Linux
    CPU: (4) x64 Intel(R) Core(TM) m3-8100Y CPU @ 1.10GHz
    Shell: 5.1.16 - /bin/bash
  Binaries:
    Node: 20.6.1 - /usr/bin/node
    Yarn: 1.22.19 - /usr/bin/yarn
    npm: 10.1.0 - /usr/bin/npm
  Languages:
    Python: 3.11.5 - /usr/bin/python
  npmPackages:
    gatsby: ^5.12.4 => 5.12.4
    gatsby-plugin-mdx: ^5.12.0 => 5.12.0
    gatsby-source-filesystem: ^5.12.0 => 5.12.0

Config Flags

N/A.