fix(adapter-nuxt): init slice library index files

[lihbr]

Resolves: https://prismic-team.slack.com/archives/C02L3FN3AJK/p1769090655683469

Description

This PR fixes the Nuxt adapter to initialize Slice library index files during project initialization, aligning its behavior with other adapters.

Previously, the adapter-nuxt was not creating index files for Slice libraries when a project was initialized. This caused issues when the project expected these files to exist for proper Slice imports.

The fix adds a new upsertSliceLibraryIndexFiles function that:

• Fetches the latest project config (important since modifySliceMachineConfig() may have updated it)
• Iterates through all configured libraries and creates their index files using the existing upsertSliceLibraryIndexFile utility

Checklist

• If my changes require tests, I added them.
• If my changes affect backward compatibility, it has been discussed.
• If my changes require an update to the CONTRIBUTING.md guide, I updated it.

Preview

N/A — This is a bug fix for project initialization, no visual changes.

How to QA ¹

1. Create a new Nuxt project with Slice Machine
2. Verify that the Slice library index file (slices/index.js or slices/index.ts) is created during initialization
3. For projects with multiple Slice libraries configured, verify all library index files are created

Footnotes

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Project	Deployment	Review	Updated (UTC)
slice-machine	Ready	Preview	Jan 23, 2026 2:19am

[xrutayisire]

Nice!

Note: Already done also in devtools repo.