Problem:
_last_resolved_tool_names in model_tools.py is a process-global variable.
When a subagent runs via delegate_task, it calls get_tool_definitions()
which overwrites this global with the subagent's restricted tool list.
After the subagent completes, the parent agent's tool list is lost, causing
execute_code to generate sandbox code with missing tool imports.

Solution:
Save and restore _last_resolved_tool_names around subagent execution in
delegate_tool.py. The save/restore is wrapped in try/except and guarded
in the finally block to handle edge cases.

Concurrency note: This fix assumes delegation runs sequentially. Each
subagent in batch mode runs in its own _run_single_child() call with its
own save/restore, so the pattern works correctly.