harbor_view stops with UnicodeEncodeError when export writes task.toml for a task.yaml whose metadata holds a lone surrogate #759

A terminal-bench task.yaml with difficulty: "n\udcff" (a YAML escape) loads with that lone surrogate in task.metadata. harbor_view (dokimi/dokimi/harnesses/harbor_export.py:71) calls _content_key(task.path, _task_toml(task)) and then export, which writes (dest / "task.toml").write_text(_task_toml(task), encoding="utf-8") at line 141 and raises UnicodeEncodeError: 'utf-8' codec can't encode character '\udcff' in position 44: surrogates not allowed (traceback ends in pathlib write_text). The key is fixed by #720; the write is not a hash, and surrogatepass there would write a task.toml that is not valid UTF-8. Fix options: refuse a metadata value that is not valid text in tbench_v1.read with a message naming the key, or drop the escape on export.

Found while fixing #720. The fix's PR is #761.