Stop advertising unreleased network volume type in 'show volumes --type'

[sammy-vastai]

Summary

Follow-up to #466. That PR hid the four dedicated network-volume commands (search network-volumes, create network-volume, list network-volume, unlist network-volume) from --help/completion, but missed a second discovery path: the already-released, always-visible vastai show volumes command takes --type local|all|network, and its help text advertised "network" as a valid value.

Confirmed against ~/workspace/vast/web: there's no dedicated "show network volume" endpoint — network volume data rides along inside the existing /volumes endpoint via that type filter, which is why this didn't show up as a separate hideable command.

Fix mirrors #466's approach: stop advertising the value in the help text; don't restrict/block it (no choices= on the argument), so --type network still works for anyone who already knows about it (internal/QA testing unaffected) — discoverability gate, not an access gate.

Test plan

• poetry run pytest tests/cli — 413 passed
• Manually verified vastai show volumes --help no longer mentions "network"