[Feature] Record state machine transition history in sandbox status

[zhangjaycee]

Feature Category

• Sandbox
• Actions
• Deployments
• SDK & API
• Envhub
• CLI
• Performance & Optimization
• Documentation & Examples

Problem Statement

When debugging sandbox lifecycle issues (e.g. a sandbox that was restarted multiple times, or stopped unexpectedly), there is no way to see the history of state transitions. Operators and SDK users must rely on scattered log entries across distributed nodes to piece together what happened and when, which is time-consuming and error-prone.

Proposed Solution

Record every state machine transition (from_state, to_state, event, timestamp) and expose the history through the existing get_status API — no new endpoints needed.

Detailed Feature Description

• Each state transition (e.g. pending→running, running→stopped, stopped→pending on restart) is automatically recorded with a timestamp.
• The transition history is stored as part of the sandbox metadata and persisted through the existing meta_store.
• The history is returned in the state_history field of the get_status response, making it available to both old and new SDK versions (old SDKs silently ignore the new field).
• History is capped at 100 entries to prevent unbounded growth in long-lived sandboxes.
• Self-transitions (e.g. stopped→stopped) are excluded to keep the history meaningful.