fix: handle OSError in save methods, add missing test, consistent logging

- Wrap save_snapshot_cache and save_last_run_date in try/except OSError, logging a warning instead of propagating the exception
- Add indent=2 to save_last_run_date for consistency
- Add warning log to load_last_run_date on read failure (matching load_snapshot_cache pattern)
- Add test_load_last_run_date_missing_key_returns_none covering valid JSON with absent key
- Remove unused asyncio, AsyncMock, and patch imports from test file

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

tests/test_incremental_snapshot.py

@@ -1,7 +1,5 @@
-import asyncio
 import json
 import pytest
-from unittest.mock import AsyncMock, patch
 
 from src.snapshot_downloader import SnapshotDownloader
 
@@ -63,6 +61,12 @@ def test_load_last_run_date_malformed_returns_none(tmp_path):
     assert d.load_last_run_date() is None
 
 
+def test_load_last_run_date_missing_key_returns_none(tmp_path):
+    d = _downloader(tmp_path)
+    (tmp_path / "last_run.json").write_text('{"date": "2025-01-01"}')
+    assert d.load_last_run_date() is None
+
+
 # --- save_last_run_date ---
 
 def test_save_last_run_date_writes_json(tmp_path):