[Security] Harden dev console assets middleware against path traversal

[gonzaloriestra]

WHY are these changes introduced?

This PR addresses a potential path traversal vulnerability in the dev console assets middleware.

WHAT is this pull request doing?

It ensures that requested asset paths are contained within the expected root directory by using a subpath check.

How to test your changes?

CI

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add

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PR created automatically by Jules for task 15539787141918695569 started by @gonzaloriestra

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