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_image_cache.py

@@ -0,0 +1,85 @@
+"""Unit: image_cache.py — check_stale / check_stale_path."""
+
+from __future__ import annotations
+
+import tempfile
+import unittest
+from datetime import datetime, timedelta, timezone
+from pathlib import Path
+from unittest.mock import patch
+
+from bot_bottle.image_cache import StaleImageError, check_stale, check_stale_path
+
+
+def _utc(**kwargs) -> datetime:
+    return datetime.now(tz=timezone.utc) - timedelta(**kwargs)
+
+
+class TestCheckStale(unittest.TestCase):
+    def _run(self, threshold: int, age_days: float) -> None:
+        created = datetime.now(tz=timezone.utc) - timedelta(days=age_days)
+        with patch("bot_bottle.image_cache.ConfigStore") as cs:
+            cs.return_value.cached_image_stale_warning_days.return_value = threshold
+            check_stale("test image", created)
+
+    def test_negative_threshold_never_raises(self):
+        # Threshold < 0 means the check is disabled — always passes.
+        self._run(threshold=-1, age_days=9999)
+
+    def test_zero_threshold_raises_immediately(self):
+        # threshold=0 means any image is stale the moment it exists.
+        with self.assertRaises(StaleImageError):
+            self._run(threshold=0, age_days=0.1)
+
+    def test_within_threshold_does_not_raise(self):
+        # Age well under threshold — should pass silently.
+        self._run(threshold=7, age_days=2)
+
+    def test_at_threshold_does_not_raise(self):
+        # Exactly at the boundary is fine (<=, not <).
+        self._run(threshold=1, age_days=0.9999)
+
+    def test_exceeds_threshold_raises(self):
+        created = datetime.now(tz=timezone.utc) - timedelta(days=3)
+        with patch("bot_bottle.image_cache.ConfigStore") as cs:
+            cs.return_value.cached_image_stale_warning_days.return_value = 1
+            with self.assertRaises(StaleImageError) as ctx:
+                check_stale("agent image 'bot-bottle:latest'", created)
+        self.assertIn("agent image", str(ctx.exception))
+        self.assertIn("day(s) old", str(ctx.exception))
+
+    def test_naive_datetime_treated_as_utc(self):
+        # check_stale calls .astimezone(utc) on the input; naive datetimes
+        # that would be interpreted as local time should still work.
+        # We can't control the local tz in a unit test, so just ensure
+        # no exception is thrown for a very recent naive datetime.
+        naive_now = datetime.utcnow()
+        with patch("bot_bottle.image_cache.ConfigStore") as cs:
+            cs.return_value.cached_image_stale_warning_days.return_value = 1
+            # Should not raise — the image is brand new.
+            check_stale("test image", naive_now)
+
+
+class TestCheckStalePath(unittest.TestCase):
+    def test_delegates_to_check_stale_with_mtime(self):
+        with tempfile.NamedTemporaryFile() as f:
+            path = Path(f.name)
+            with patch("bot_bottle.image_cache.check_stale") as mock_check:
+                check_stale_path("some artifact", path)
+            mock_check.assert_called_once()
+            label, dt = mock_check.call_args.args
+            self.assertEqual("some artifact", label)
+            self.assertIsInstance(dt, datetime)
+            self.assertIsNotNone(dt.tzinfo)
+
+    def test_raises_stale_for_old_file(self):
+        with tempfile.NamedTemporaryFile() as f:
+            path = Path(f.name)
+            with patch("bot_bottle.image_cache.ConfigStore") as cs:
+                cs.return_value.cached_image_stale_warning_days.return_value = 0
+                with self.assertRaises(StaleImageError):
+                    check_stale_path("cached artifact", path)
+
+
+if __name__ == "__main__":
+    unittest.main()