Fix sandbox startup polling through transient Stopped states

[arkaly24-cloud]

Changes

• Keep polling through transient Stopped states during sandbox startup and retry start requests at most once every 15 seconds.
• Preserve terminal failures, timeout and cancellation handling, and stop-wait behavior.
• Add deterministic polling-loop tests, update the SSH lifecycle regression test, and add a CLI changelog entry.

Why

An accepted start request can be followed by a transient Stopped status. Treating the first such poll as terminal makes databricks sandbox start and databricks sandbox ssh fail before the sandbox can recover.

Tests

• GOTOOLCHAIN=go1.26.8 go test ./cmd/sandbox -count=1
• go test -race ./cmd/sandbox -count=1
• go vet ./cmd/sandbox
• GOTOOLCHAIN=go1.26.8 go tool -modfile=tools/go.mod golangci-lint run --allow-parallel-runners -j=4 ./cmd/sandbox/...
• python3.12 tools/validate_nextchanges.py
• Regression reproduced before the fix; the new virtual-time tests also pass across 25 consecutive runs.
• Full root/tools Go unit suites (11,935 passed, 126 skipped), doctests, ./task checks, Go/YAML formatting, and full Go lint passed with Go 1.26.8.

Full acceptance testing stopped at its prerequisite check because this machine has jq 1.6, while the suite requires 1.7 or newer. Python formatting/lint could not run because downloading the pinned Ruff version failed on PyPI DNS resolution. No unrelated code changes were made for these environment limitations.

This PR was written with Codex.

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