lakebox: add start command, tab completion, fail-fast on unknown sandbox

[akshaysingla-db]

Three small additions that close gaps surfaced testing the gateway-host PR end-to-end.

1. databricks lakebox start <id> — wraps the existing StartSandbox RPC. Symmetric to stop. Useful for pre-warming a sandbox before connecting (the gateway also auto-starts on ssh, so this is mostly for scripting).

2. Tab completion for sandbox IDs on every command that takes one (ssh / status / stop / start / delete / config / set-default) and key hashes on ssh-key delete. Best-effort: completion silently produces no suggestions when the API call fails so the shell stays usable. Standard Cobra ValidArgsFunction pattern, surfaced via databricks completion bash|zsh|fish like every other Cobra CLI.

3. databricks lakebox ssh <unknown-id> now fails with no lakebox named "..." — databricks lakebox list shows available IDs instead of falling through to ssh and surfacing Permission denied (publickey) from the gateway. Implemented by making the existing pre-ssh api.get (added in lakebox: read SSH gateway host from Sandbox response, cache per profile #5292 for gateway discovery) treat apierr.ErrNotFound as fatal. Non-NotFound errors still fall through so transient API hiccups don't block a connection the gateway can still route.

Co-authored-by: Isaac

Changes

Why

Tests

[github-actions]

Waiting for approval

Based on git history, these people are best suited to review:

• @pietern -- recent work in cmd/lakebox/
• @shuochen0311 -- recent work in cmd/lakebox/

Eligible reviewers: @andrewnester, @anton-107, @denik, @renaudhartert-db, @shreyas-goenka, @simonfaltum

_{Suggestions based on git history. See OWNERS for ownership rules.}