[fix] Add server module mock to unit tests for PromptServer import

Updated test to mock server module preventing import errors from the new PromptServer usage in GetImageSize node. Uses direct import pattern consistent with rest of codebase.

tests-unit/comfy_extras_test/image_stitch_test.py

@@ -5,7 +5,10 @@ from unittest.mock import patch, MagicMock
 mock_nodes = MagicMock()
 mock_nodes.MAX_RESOLUTION = 16384
 
-with patch.dict('sys.modules', {'nodes': mock_nodes}):
+# Mock server module for PromptServer
+mock_server = MagicMock()
+
+with patch.dict('sys.modules', {'nodes': mock_nodes, 'server': mock_server}):
     from comfy_extras.nodes_images import ImageStitch