[checkpoint] Restore nemo_fabric_agent sessions #3283

Parent

Child of #3222 and #3024. Depends on #3275.

Runtime state

responses_api_agents/nemo_fabric_agent/app.py owns Fabric turn/session state, workspace, sandbox handle, and pending operation cursor.

Adapter requirements

Persist the logical execution-to-runtime mapping, provider/version and physical generation, sandbox snapshot/reference, image/spec and mount fingerprints, agent or verifier cursor, and result-manifest identity. Rebind a replacement runtime atomically and fence the old handle. Never claim that a filesystem-only copy restores unsupported live process state.

Acceptance criteria

• Fabric emits explicit safe boundaries and exports typed session state paired with the matching workspace snapshot.
• Mutate runtime state, commit, replace both application and runtime process where supported, restore, and compare the next operation/result.
• Stale handles and delayed old-runtime results are rejected.
• Partial restore activates no execution mapping.
• Duplicate retirement and failed-provider cleanup release the restored runtime exactly once.