Hide unreleased network-volume commands from --help

[sammy-vastai]

Summary

Network volumes aren't released yet, but their CLI commands were fully visible in vastai --help and tab completion — search network-volumes, create network-volume, list network-volume, unlist network-volume.

Adds a general-purpose "hidden command" gate, independent of and unrelated to the client/host work in #462/#463:

• HIDDEN_COMMANDS (name-based registry) + an explicit hidden= kwarg on @parser.command, mirroring the existing COMMAND_OVERRIDES-style pattern in vastai/cli/parser.py.
• Discoverability gate only, not an access gate — a hidden command still runs if typed directly (e.g. vastai search network-volumes ...), so internal testing is unaffected.
• Applied to all four network-volume commands.

Found while manually testing #462 — noticed search network-volumes in --help but its own usage: string has an unrelated pre-existing typo (says search network volumes, missing the hyphen, present on master too). Not fixed here since it's a separate, small, unrelated issue — happy to do it as a follow-up if wanted.

Test plan

• poetry run pytest tests/cli — 375 passed
• Manually verified: none of the four commands appear in vastai --help or tab completion; all four still parse and run when typed directly

[Copilot]

Pull request overview

This PR introduces a general-purpose “hidden command” mechanism in the CLI parser to remove unreleased commands from vastai --help output and tab completion, while keeping those commands executable when invoked directly (discoverability gate only).

Changes:

• Adds HIDDEN_COMMANDS + is_hidden_command() and a hidden= kwarg on @parser.command to mark commands as hidden.
• Filters hidden subparsers out of both grouped --help rendering and completion map generation.
• Adds tests covering the hidden-command registry behavior, decorator propagation, help suppression, and direct parsing of a hidden command.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
vastai/cli/parser.py	Implements hidden-command metadata and filters hidden commands out of help/completion paths.
tests/cli/test_parser.py	Adds unit/sanity tests to ensure hidden commands are not discoverable but remain runnable.

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[vastzuby]

yeah anything network volume or cluster related should be hidden