feat: add snapshot cache and state file I/O methods

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_incremental_snapshot.py

@@ -0,0 +1,72 @@
+import asyncio
+import json
+import pytest
+from unittest.mock import AsyncMock, patch
+
+from src.snapshot_downloader import SnapshotDownloader
+
+
+def _downloader(tmp_path):
+    return SnapshotDownloader(output_dir=str(tmp_path), api_key="test-key")
+
+
+# --- load_snapshot_cache ---
+
+def test_load_snapshot_cache_missing(tmp_path):
+    assert _downloader(tmp_path).load_snapshot_cache() == []
+
+
+def test_load_snapshot_cache_returns_data(tmp_path):
+    d = _downloader(tmp_path)
+    snapshots = [{"id": "1", "notes": "hello"}]
+    (tmp_path / "snapshots_cache.json").write_text(json.dumps(snapshots))
+    assert d.load_snapshot_cache() == snapshots
+
+
+def test_load_snapshot_cache_malformed_returns_empty(tmp_path):
+    d = _downloader(tmp_path)
+    (tmp_path / "snapshots_cache.json").write_text("not json{{{")
+    assert d.load_snapshot_cache() == []
+
+
+def test_load_snapshot_cache_non_list_returns_empty(tmp_path):
+    d = _downloader(tmp_path)
+    (tmp_path / "snapshots_cache.json").write_text('{"key": "val"}')
+    assert d.load_snapshot_cache() == []
+
+
+# --- save_snapshot_cache ---
+
+def test_save_snapshot_cache_writes_json(tmp_path):
+    d = _downloader(tmp_path)
+    snapshots = [{"id": "1"}, {"id": "2"}]
+    d.save_snapshot_cache(snapshots)
+    data = json.loads((tmp_path / "snapshots_cache.json").read_text())
+    assert data == snapshots
+
+
+# --- load_last_run_date ---
+
+def test_load_last_run_date_missing(tmp_path):
+    assert _downloader(tmp_path).load_last_run_date() is None
+
+
+def test_load_last_run_date_returns_date(tmp_path):
+    d = _downloader(tmp_path)
+    (tmp_path / "last_run.json").write_text('{"last_date_to": "2025-01-01"}')
+    assert d.load_last_run_date() == "2025-01-01"
+
+
+def test_load_last_run_date_malformed_returns_none(tmp_path):
+    d = _downloader(tmp_path)
+    (tmp_path / "last_run.json").write_text("not json")
+    assert d.load_last_run_date() is None
+
+
+# --- save_last_run_date ---
+
+def test_save_last_run_date_writes_json(tmp_path):
+    d = _downloader(tmp_path)
+    d.save_last_run_date("2025-06-01")
+    data = json.loads((tmp_path / "last_run.json").read_text())
+    assert data == {"last_date_to": "2025-06-01"}