fix(a2a): detach sandbox runs from client streams

[drewstone]

Summary

• Use the Sandbox SDK dispatchPrompt + exact session execution controls for A2A task runs.
• Persist the accepted sandbox/environment, session, and execution IDs on the task execution fence.
• Treat response disconnects as observation-only; tasks/resubscribe replays the exact run and tasks/get reconciles its result before lease failure.
• Restrict remote interruption to explicit tasks/cancel, and fail closed when production adapters lack durable controls.

This follows the Sandbox SDK README “Resume a session” contract and its session.events, session.result, and session.interrupt primitives.

Proof

• pnpm typecheck
• pnpm test -- --reporter=dot — 24 files, 392 tests passed
• Focused disconnect/resubscribe, exact cancel, expired-run recovery, and webhook regression tests included.