fix(tests): thread cleanup on assertion failure, reduce oversized image

- test_health_monitor.py: wrap start_monitoring calls in try/finally so
  the background thread is always stopped even when an assertion fails
- test_scroll_helper.py: reduce 50,000px test image to 5,000px to avoid
  unnecessary memory pressure on Raspberry Pi

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

test/test_health_monitor.py

@@ -280,9 +280,11 @@ class TestGetRecoverySuggestions:
 class TestMonitorLifecycle:
     def test_start_monitoring(self, monitor):
         monitor.start_monitoring()
-        assert monitor._monitor_thread is not None
-        assert monitor._monitor_thread.is_alive()
-        monitor.stop_monitoring()
+        try:
+            assert monitor._monitor_thread is not None
+            assert monitor._monitor_thread.is_alive()
+        finally:
+            monitor.stop_monitoring()
 
     def test_stop_monitoring(self, monitor):
         monitor.start_monitoring()
@@ -292,10 +294,12 @@ class TestMonitorLifecycle:
 
     def test_double_start_no_duplicate_threads(self, monitor):
         monitor.start_monitoring()
-        thread1 = monitor._monitor_thread
-        monitor.start_monitoring()  # should be idempotent
-        assert monitor._monitor_thread is thread1
-        monitor.stop_monitoring()
+        try:
+            thread1 = monitor._monitor_thread
+            monitor.start_monitoring()  # should be idempotent
+            assert monitor._monitor_thread is thread1
+        finally:
+            monitor.stop_monitoring()
 
     def test_register_health_check(self, monitor):
         callback = MagicMock()