fix: restore Codecov slug to fix Repository not found error

[PierreBrisorgueil]

Fix

Restores the slug: pierreb-devkit/Node parameter removed in a previous PR on Copilot's suggestion.

Without it, codecov-action@v5 cannot resolve the repository and throws:
```
Upload failed: {"message":"Repository not found"}
```

The slug is required for Codecov to correctly identify the target repository.

[Copilot]

Pull request overview

This PR restores the slug parameter to the Codecov upload step in the CI workflow to fix a "Repository not found" error. The parameter was previously removed in another PR but is required for codecov-action@v5 to correctly identify the target repository.

Changes:

• Added slug: pierreb-devkit/Node parameter to the codecov-action@v5 step in CI workflow

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