test: expand npm CLI coverage

[JacobOptimiza]

Summary

• add real-process coverage for the npm CLI help, version, unknown-command, and manifest-error paths
• expand manifest and installation-path assertions without changing production code
• keep the npm package, workflows, release, and distribution configuration unchanged

Validation

• node --test "tests/npm/**/*.test.mjs"
• c8@12.0.0 (temporary): 90.39% statements
• Node built-in coverage cross-check
• cargo fmt/check/test/clippy/deny
• PowerShell parser, PSScriptAnalyzer, and Pester
• git diff --check