[checkpoint] Define a checkpointable environment protocol for OpenEnv #3245

Parent

Child of #3219 and #3024.

Problem and state owner

resources_servers/openenv/app.py owns arbitrary live environment instances plus accumulated reward, step count, done state, and last observation.

Proposed adapter

Define an explicit environment export/import capability and snapshot only implementations that satisfy it; never pickle arbitrary Any environments or silently reset them.

Implement explicit checkpoint opt-in, route classification, export/restore/retire hooks, execution-to-session binding, revision reporting, atomic replacement under attempt N+1, and stale-attempt fencing.

Acceptance criteria

• Checkpoint-capable MCP and non-MCP test environments continue exactly; unsupported environments reject enablement or prepare.
• Checkpoint after multiple mutations, replace the server process, restore, and compare the next operation with an uninterrupted execution.
• Corrupt or incomplete bulk state activates no sessions.
• Attempt N cannot mutate or retire restored attempt N+1.
• Terminal cleanup is idempotent and releases restored state exactly once.