fix(volume): resolve Network and Instance lookup fallbacks in volume ls (fixes #597)

[mahendrarathore1742]

### Summary
Fixes `civo volume ls` lookups for networks and instances so that unmatched items

safely fall back to raw IDs instead of showing unrelated names or breaking table column
alignment.

### Changes
- Replaced loop variable re-assignments with local fallback variables for network and

instance lookups.
- Ensured instance_id column is always appended once per row for consistent table
alignment.
- Added unit tests in cmd/volume/volume_list_test.go to verify matched and unmatched
fallback behavior.

Fixes #597