[checkpoint] Define checkpoint adapters for ns_tools sessions

Parent

Child of #3219 and #3024.

Problem and state owner

resources_servers/ns_tools/app.py owns ToolManager-owned stateful tool sessions and per-session timing data used by verification.

Proposed adapter

Add a ToolManager snapshot/reference API for supported tools, preserve exact timing records, and reject arbitrary externally mutating tools without a safe contract.

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

• DirectPython state and timing totals survive process replacement; unsupported stateful tools fail checkpoint admission explicitly.
• 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.

Start with resources_servers/ns_tools/app.py and review the parent issues #3219 and #3024 to understand ToolManager-owned session state and timing data. Trace the checkpoint entry points and define completion against the listed acceptance criteria: safe export and restore, rejection of unsupported state, atomic replacement, stale-attempt fencing, and idempotent cleanup.