sandbox: cap API and SSH timeouts so unenabled regions fail fast

[akshaysingla-db]

Summary

• In regions where the sandbox manager service isn't deployed, the gateway silently holds the connection rather than returning a structured error, so databricks sandbox <anything> would hang for ~60s on the API call and another ~75s in ssh before giving up.
• Tighten the per-request timeout on every sandbox HTTP call to 10s (via context.WithTimeout inside a new do wrapper on *sandboxAPI), and add -o ConnectTimeout=10 to the ssh argv.
• Translate context.DeadlineExceeded from the API path into "sandbox API timed out after 10s — this region may not have sandbox enabled, or the manager is unreachable", so the user gets an actionable hint instead of a raw deadline error.

Long flows (cold start, etc.) are made of many short calls polled in a loop, so each call still has its own 10s budget while the outer wait loop's own deadline (startWaitTimeout = 10m) keeps governing the overall flow.

Considered routing through the SDK's Config.HTTPTimeoutSeconds instead, but that path rewrites a timeout into a freshly-allocated url.Error whose inner error no longer satisfies errors.Is(err, context.DeadlineExceeded) — making clean error detection require string matching, which the repo style guide forbids. The context.WithTimeout approach composes fine with the SDK's 60s default backstop.

Test plan

• go test ./cmd/sandbox/... — new TestSandboxAPIDoTranslatesTimeout exercises the wrapper against an httptest server that hangs and asserts both the "timed out" string and the region hint; existing tests pass unchanged.
• TestBuildSSHArgsBaseFlags now asserts ConnectTimeout=10 is in the argv.
• Manual: invoke databricks sandbox list against a profile in a region without the manager and confirm the clear timeout message arrives in ~10s instead of hanging.

This pull request and its description were written by Isaac.

[github-actions]

Waiting for approval

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

• @pietern -- recent work in cmd/sandbox/

Eligible reviewers: @shuochen0311

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