Fix MDX page directory boundary detection

[NgoQuocViet2001]

Changes

• Require a directory boundary when deciding whether an MDX file is under src/pages.
• Prevent sibling directories such as src/pages-old and src/pages2 from receiving page-only automatic charset injection in both the unified and Sätteri processors.

Testing

• Added focused coverage for real pages and similarly prefixed sibling directories across both processor paths.
• Ran: pnpm -C packages/integrations/mdx test
• Ran: pnpm -C packages/integrations/mdx build
• Ran: pnpm lint:ai

Docs

No docs changes. This corrects internal page classification without changing the public API.

[changeset-bot]

🦋 Changeset detected

Latest commit: f63144a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 47 packages

Name	Type
@astrojs/mdx	Patch
@test/astro-cloudflare	Patch
@e2e/content-collections	Patch
@e2e/csp-server-islands	Patch
@e2e/preact-component	Patch
@e2e/preact-lazy-component	Patch
@e2e/react-component	Patch
@e2e/server-islands	Patch
@e2e/solid-component	Patch
@e2e/svelte-component	Patch
@e2e/vue-component	Patch
@performance/mdx	Patch
@test/astro-assets-prefix	Patch
@test/astro-basic	Patch
@test/content-with-spaces-in-folder-name	Patch
@test/content-collections-base	Patch
@test/content-collections-mutation	Patch
@test/content-collections	Patch
@test/content-intellisense	Patch
@test/content-layer-remark-plugins	Patch
@test/content-layer	Patch
@test/content-ssr-integration	Patch
@test/content-static-paths-integration	Patch
@test/content	Patch
@test/core-image-deletion	Patch
@test/core-image-svg	Patch
@test/incremental-build-content	Patch
@test/incremental-build	Patch
@test/jsx-queue-rendering	Patch
@test/lightningcss-css-modules-content	Patch
@test/partials	Patch
@test/server-islands-ssr	Patch
@test/slots-preact	Patch
@test/slots-react	Patch
@test/slots-solid	Patch
@test/slots-svelte	Patch
@test/slots-vue	Patch
@test/special-chars-in-component-imports	Patch
@test/tailwindcss	Patch
@test/astro-cloudflare-prerender-styles	Patch
@test/astro-cloudflare-vite-plugin	Patch
@test/content-layer-rendering	Patch
@test/mdx-css-head-mdx	Patch
@test/image-remark-imgattr	Patch
@test/mdx-astro-container-escape	Patch
@test/mdx-frontmatter-injection	Patch
astro-benchmark	Patch

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