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>

src/snapshot_downloader.py

@@ -553,8 +553,11 @@ class SnapshotDownloader:
 
     def save_snapshot_cache(self, snapshots: List[Dict[str, Any]]) -> None:
         cache_file = self.output_dir / "snapshots_cache.json"
-        with open(cache_file, "w", encoding="utf-8") as f:
-            json.dump(snapshots, f, indent=2, default=str)
+        try:
+            with open(cache_file, "w", encoding="utf-8") as f:
+                json.dump(snapshots, f, indent=2, default=str)
+        except OSError as e:
+            self.logger.warning(f"Could not write snapshot cache: {e}")
 
     def load_last_run_date(self) -> Optional[str]:
         state_file = self.output_dir / "last_run.json"
@@ -565,12 +568,16 @@ class SnapshotDownloader:
                 data = json.load(f)
             return data.get("last_date_to")
         except (json.JSONDecodeError, OSError):
+            self.logger.warning("Could not read last run date; will do full fetch")
             return None
 
     def save_last_run_date(self, date: str) -> None:
         state_file = self.output_dir / "last_run.json"
-        with open(state_file, "w", encoding="utf-8") as f:
-            json.dump({"last_date_to": date}, f)
+        try:
+            with open(state_file, "w", encoding="utf-8") as f:
+                json.dump({"last_date_to": date}, f, indent=2)
+        except OSError as e:
+            self.logger.warning(f"Could not write last run date: {e}")
 
     async def generate_html_file(
         self, snapshots: List[Dict[str, Any]], date_from: str, date_to: str

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):