test: add integration tests for checkpoint store (stderr capture + large file skip)

Author: Simon FREYBURGER

tests/test_checkpoint_store.py

@@ -0,0 +1,61 @@
+"""Integration tests for checkpoint store: stderr capture + large file skip."""
+from __future__ import annotations
+
+import pytest
+
+import checkpoint.store as store
+
+
+@pytest.fixture(autouse=True)
+def isolate_store(tmp_path, monkeypatch):
+    """Redirect checkpoint root to tmp_path and reset global state."""
+    monkeypatch.setattr(store, "_checkpoints_root", lambda: tmp_path / "checkpoints")
+    store.reset_file_versions()
+
+
+def test_large_file_skipped_and_logged_to_stderr(tmp_path, monkeypatch, capsys):
+    monkeypatch.setattr(store, "_MAX_FILE_SIZE", 50)
+    big_file = tmp_path / "big.txt"
+    big_file.write_bytes(b"x" * 100)
+
+    result = store.track_file_edit("test-session", str(big_file))
+
+    assert result is None
+    captured = capsys.readouterr()
+    assert "[checkpoint] skipping large file" in captured.err
+    assert "100 bytes" in captured.err
+    assert captured.out == ""
+
+
+def test_normal_file_backed_up(tmp_path, capsys):
+    small_file = tmp_path / "small.txt"
+    content = b"hello world"
+    small_file.write_bytes(content)
+
+    result = store.track_file_edit("test-session", str(small_file))
+
+    assert result is not None
+    backup_dir = tmp_path / "checkpoints" / "test-session" / "backups"
+    backup_path = backup_dir / result
+    assert backup_path.exists()
+    assert backup_path.read_bytes() == content
+    captured = capsys.readouterr()
+    assert captured.err == ""
+
+
+def test_backup_failure_logged_to_stderr(tmp_path, monkeypatch, capsys):
+    normal_file = tmp_path / "normal.txt"
+    normal_file.write_bytes(b"some data")
+
+    def failing_copy(*args, **kwargs):
+        raise PermissionError("access denied")
+
+    monkeypatch.setattr(store.shutil, "copy2", failing_copy)
+
+    result = store.track_file_edit("test-session", str(normal_file))
+
+    assert result is None
+    captured = capsys.readouterr()
+    assert "[checkpoint] backup failed" in captured.err
+    assert "access denied" in captured.err
+    assert captured.out == ""