fix(session): restore state when file-cap archive fails

tests/session/test_session_store.py

@@ -1,5 +1,7 @@
 from unittest.mock import MagicMock
 
+import pytest
+
 import nanobot.session as session_api
 from nanobot.session import Session, SessionManager
 from nanobot.session.manager import FILE_MAX_MESSAGES, SessionStore
@@ -77,3 +79,31 @@ def test_manager_applies_file_cap_before_store_save(tmp_path) -> None:
     assert len(session.messages) == FILE_MAX_MESSAGES
     archiver.assert_called_once()
     store.save.assert_called_once_with(session, fsync=False)
+
+
+def test_manager_retries_file_cap_archive_after_failure(tmp_path) -> None:
+    store = MagicMock(spec=SessionStore)
+    archiver = MagicMock(side_effect=[RuntimeError("history unavailable"), None])
+    manager = SessionManager(tmp_path, store=store)
+    manager.set_file_cap_archiver(archiver)
+    session = Session(
+        key="cli:retry-large",
+        messages=[
+            {"role": "user", "content": str(index)}
+            for index in range(FILE_MAX_MESSAGES + 1)
+        ],
+    )
+
+    with pytest.raises(RuntimeError, match="history unavailable"):
+        manager.save(session)
+
+    assert len(session.messages) == FILE_MAX_MESSAGES + 1
+    store.save.assert_not_called()
+
+    manager.save(session)
+
+    assert len(session.messages) == FILE_MAX_MESSAGES
+    assert archiver.call_count == 2
+    assert archiver.call_args_list[0].args[0][0]["content"] == "0"
+    assert archiver.call_args_list[1].args[0][0]["content"] == "0"
+    store.save.assert_called_once_with(session, fsync=False)