fix(async-agent): improve save_checkpoint directory creation and handle null tasks in load_checkpoint

[santhreal]

In chapter4/async-agent/runtime.py, save_checkpoint raised FileNotFoundError when target parent directories did not exist, and load_checkpoint raised TypeError during log formatting when a checkpoint JSON had "tasks": null.

This fix automatically creates parent directories via os.makedirs and uses len(data.get('tasks') or []). Includes regression tests in chapter4/async-agent/test_checkpoint_robustness.py.

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Reviewing files that changed from the base of the PR and between ca666c1 and c88bc28.

📒 Files selected for processing (2)

• chapter4/async-agent/runtime.py
• chapter4/async-agent/test_checkpoint_robustness.py

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[bojieli]

The null-tasks path is still broken before the guarded log line: load_checkpoint calls self.tasks.restore(data.get("tasks", [])), which passes None when the JSON contains "tasks": null. TaskManager.restore then iterates over None and raises TypeError. The included test uses MagicMock for runtime.tasks, so it masks this failure. Please normalize once (for example, tasks = data.get("tasks") or []), pass that value to restore and use it for the count, then exercise load_checkpoint with a real TaskManager in the regression test.