MDX inherits global `markdown.processor` even with `extendMarkdownConfig: false`

Astro Info

Astro                    v6.4.5
Vite                     v7.3.5
Node                     v22.22.0
System                   Linux (x64)
Package Manager          npm
Output                   static
Adapter                  none
Integrations             @astrojs/mdx (v6.0.3)

If this issue only occurs in one browser, which browser is a problem?

No response

Describe the Bug

The Markdown guide states that with extendMarkdownConfig: false, MDX uses the "Default unified() processor used with no plugins".

However, @astrojs/mdx always falls back to the global processor, regardless of the flag. In astro:config:done (packages/integrations/mdx/src/index.ts):

// remark/rehype plugins, remarkRehype, gfm, smartypants are all gated:
const markdownConfig = extendMarkdownConfig ? config.markdown : markdownConfigDefaults;
if (extendMarkdownConfig && isUnifiedProcessor(processor)) { /* ... */ }
if (extendMarkdownConfig && isSatteriProcessor(processor)) { /* ... */ }

// ...but the processor itself is not:
const processor = partialMdxOptions.processor ?? config.markdown.processor;

As a result, when the global config uses a non-default processor (e.g. satteri() from @astrojs/markdown-satteri with hastPlugins), those processor-level plugins are applied to .mdx files even though
extendMarkdownConfig: false is set.

Note: the JSDoc on the processor option of mdx() says it "Defaults to config.markdown.processor" unconditionally, so it's possible this is the intended behavior — in that case the docs should be updated instead, since they currently promise a default processor with no plugins. Either way the docs and the implementation currently contradict each other.

If the implementation is the side to fix, I'm happy to send a PR. A possible fix is to gate the fallback and use a fresh default processor:

const processor =
  partialMdxOptions.processor ??
  (extendMarkdownConfig ? config.markdown.processor : unified());

What's the expected result?

With extendMarkdownConfig: false (and no processor passed to mdx()), .mdx files should be rendered with the default unified() processor with no plugins, as documented — independent of the global markdown.processor.

Link to Minimal Reproducible Example

https://stackblitz.com/edit/withastro-astro-aujem2cb?file=astro.config.mjs

Participation

• I am willing to submit a pull request for this issue.