fix(arborist): retain explicit optional peers with --no-save

[wangxpych]

Summary

• retain an explicitly requested optional peer during --no-save reification
• treat that package as a temporary dependency root when recalculating dependency flags
• cover both the requested package and its transitive dependencies with a regression test

Problem

With an existing lockfile, a package already declared as an optional peer can be resolved for an explicit npm install --no-save <package> request and then marked extraneous during dependency flag recalculation. The pruning pass removes it again, so npm reports the project as up to date.

Testing

• npm test --workspace @npmcli/arborist
• reproduced with the package manifest from [BUG] install --no-save does not install anything #9024; after the fix eslint-plugin-jest@29.16.6 is installed while package.json and package-lock.json hashes remain unchanged

Fixes #9024

[wangxpych]

Closing because I’m stepping back from maintaining this contribution. Thanks for your time.