gitlab-runner: TTL-based cleanup of stale suspended environments (Fleeting-based executors)

MRs:

• !7640 (merged)
• !7644 (merged)
• !7645 (merged)
• !7646 (merged)

Implement a background loop in the runner that tears down suspended environments whose TTL has expired. This is the safety net for crashed workflows, abandoned sessions, and orchestrator bugs.

• Track suspension timestamp per acquisition in the runner's persisted state.
• Periodically check for expired environments (configurable TTL, default 1 week).
• On expiry: call scaler.Terminate to release the acquisition and tear down the instance.
• Must survive runner restarts (suspension state is already persisted to disk).
• Log each expiry for operator visibility.