#19098: fix(cli): improve update version check messaging

## Summary When npm registry is unreachable and `targetVersion` is null, the update command incorrectly treats it as a 'downgrade' and shows 'Downgrading from X to (unknown)' which is confusing. ## Changes - Show 'Could not resolve version for \<tag\>' when npm registry is unreachable (instead of downgrade message) - Show 'Already up to date (version)' when current matches target version - Only trigger downgrade confirmation for actual downgrades (`cmp > 0`), not unknown versions - Preserve channel persistence and plugin updates even when version resolution fails ## Testing Added 3 test cases: - `reports unresolved channel version without downgrade messaging` - `logs already up to date when current and target versions match` - `keeps channel persistence when target version resolution fails` All 24 tests passing. Closes #8450 Closes #10690