fix(pairing): restore durable atomic writes

tests/utils/test_helpers.py

@@ -1,6 +1,9 @@
+from pathlib import Path
+
 import tiktoken
 
-from nanobot.utils.helpers import split_message, truncate_text_to_tokens
+from nanobot.utils import helpers
+from nanobot.utils.helpers import _write_text_atomic, split_message, truncate_text_to_tokens
 
 
 def test_split_message_no_code_blocks_unchanged():
@@ -31,3 +34,44 @@ def test_truncate_text_to_tokens_non_positive_budget_returns_text():
     text = "anything"
 
     assert truncate_text_to_tokens(text, 0) == text
+
+
+def test_write_text_atomic_fsyncs_file_and_parent_directory(
+    tmp_path: Path, monkeypatch
+) -> None:
+    target = tmp_path / "pairing.json"
+    fsync_calls: list[int] = []
+    closed_fds: list[int] = []
+
+    def fake_fsync(fd: int) -> None:
+        fsync_calls.append(fd)
+
+    monkeypatch.setattr(helpers.os, "fsync", fake_fsync)
+    monkeypatch.setattr(helpers.os, "open", lambda path, flags: 12345)
+    monkeypatch.setattr(helpers.os, "close", lambda fd: closed_fds.append(fd))
+
+    _write_text_atomic(target, '{"approved": {}}')
+
+    assert target.read_text(encoding="utf-8") == '{"approved": {}}'
+    assert len(fsync_calls) == 2
+    assert fsync_calls[0] != 12345
+    assert fsync_calls[1] == 12345
+    assert closed_fds == [12345]
+
+
+def test_write_text_atomic_keeps_file_when_directory_fsync_is_unsupported(
+    tmp_path: Path, monkeypatch
+) -> None:
+    target = tmp_path / "pairing.json"
+    fsync_calls: list[int] = []
+
+    def fake_open(path, flags):
+        raise OSError("directory fsync unsupported")
+
+    monkeypatch.setattr(helpers.os, "fsync", lambda fd: fsync_calls.append(fd))
+    monkeypatch.setattr(helpers.os, "open", fake_open)
+
+    _write_text_atomic(target, '{"pending": {}}')
+
+    assert target.read_text(encoding="utf-8") == '{"pending": {}}'
+    assert len(fsync_calls) == 1