fix: exclude unrelated optional peers from workspace installs

[Sakshamm-Goyal]

Summary

• stop filtered workspace reification from treating peerOptional edges as requirements on their own
• keep optional peer providers when the root or selected workspace explicitly depends on them
• add regression coverage for both the out-of-scope and explicit-provider cases

This fixes scoped npm ci --workspace installs pulling in a package that is only declared by an unselected workspace and then reporting it as extraneous.

Validation

• reproduced the issue with the reporter repository on npm CLI latest: sass-embedded@1.100.0 was installed and reported extraneous
• reran the same repository with this patch: sass-embedded is absent while vite and the selected pkg-b workspace remain installed
• node . run test -w @npmcli/arborist (59 suites, 100% statement/branch/function/line coverage, workspace lint passes)
• focused regression verifies an explicit selected-workspace provider remains installed

Fixes #9847