Clarify Fastify middleware asset handling

[NgThachThanh]

Description

Clarifies the Fastify middleware example for @astrojs/node so static client assets are handled by Fastify, while page/API/error routes are handled by Astro.

This adds a guard for files in dist/client/ and generated /_astro/ assets before calling the Astro SSR handler.

Closes #13864.

Tests

• pnpm exec prettier --check src/content/docs/en/guides/integrations-guide/node.mdx
• pnpm run check

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Warnings reference

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

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