Closes #33684

Problem

When using ShellToolMiddleware with human-in-the-loop (HIL) interrupts, the persistent shell session is lost on resume:

• _SessionResources was stored in agent state → fails msgpack serialization
• No mechanism to restore the shell session after interruption
• Result: shell tool fails or starts a new session on resume

Fix

This PR fully resolves the HIL session loss by:

• Caching the shell session in the middleware instance (_session, _tempdir)
• Saving only serializable config to checkpoint.metadata:

  "shell_session": {
      "workspace": "...",
      "command": [...],
      "env": {...},
      "policy": {...}
  }

Restoring session via a new restore_shell graph node when resume_from == "interrupt"
Adding to_dict() / from_dict() to BaseExecutionPolicy for safe serialization