[Feature] Support transactional sandbox dependency groups

[matrix72c]

Summary

• extend SandboxSpec with one-level dependency groups and an optional provisioner
• create and health-check dependencies before the primary sandbox
• roll back the whole group in reverse order when creation, health checks, or provisioning fail
• retry group acquisition transactionally and clean up returned members on cancellation
• keep the public pool API backward-compatible by exposing only the primary client
• apply sandbox creation rate limiting to every physical sandbox

Tests

• pytest -q tests/rl/test_sandbox_pool.py: 8 passed
• ruff check: passed
• ruff format --check: passed
• git diff --check: passed