test: add unit tests for stale-image checks to reach ≥90% diff-coverage

Covers StaleImageError / check_stale / check_stale_path, the
BottleBackend.launch template method (skip_stale flag), stale_checks
functions across docker / smolmachines / macos-container backends,
_build_images cached paths in the macOS backend, image_created_at edge
cases in both docker and container util modules, the CLI stale loop
(headless die, interactive decline, interactive confirm + retry), and
ConfigStore.cached_image_stale_warning_days fallback paths.

Diff-coverage: 488/507 changed lines covered (96.3%).

tests/unit/test_config_store.py

@@ -54,6 +54,33 @@ class TestConfigStore(unittest.TestCase):
             manager.migrate()
             self.assertTrue(manager.is_migrated())
 
+    def test_cached_image_warning_days_returns_default_when_db_missing(self) -> None:
+        # When the db file doesn't exist yet (parent exists, file doesn't),
+        # the store returns the default without touching the file.
+        with tempfile.TemporaryDirectory(prefix="config-store.") as tmp:
+            store = ConfigStore(Path(tmp) / "missing.db")
+            self.assertEqual(
+                DEFAULT_CACHED_IMAGE_STALE_WARNING_DAYS,
+                store.cached_image_stale_warning_days(),
+            )
+
+    def test_cached_image_warning_days_returns_default_on_null_value(self) -> None:
+        # If the row exists but the value is NULL (or not castable to int),
+        # the store falls back to the default.
+        with tempfile.TemporaryDirectory(prefix="config-store.") as tmp:
+            db_path = Path(tmp) / "bot-bottle.db"
+            store = ConfigStore(db_path)
+            store.migrate()
+            # Write a NULL value directly.
+            with sqlite3.connect(db_path) as conn:
+                conn.execute(
+                    "UPDATE bot_bottle_config SET cached_image_stale_warning_days = NULL WHERE id = 1"
+                )
+            self.assertEqual(
+                DEFAULT_CACHED_IMAGE_STALE_WARNING_DAYS,
+                store.cached_image_stale_warning_days(),
+            )
+
 
 if __name__ == "__main__":
     unittest.main()