Add auto-initialization to MSS and DXcam engines to fix WGC multi-monitor issue

- MSS and DXcam now auto-initialize on first capture_display() call, matching WGC behavior
- Remove engine.initialize() call from test endpoint to prevent WGC from initializing monitor 0 unnecessarily
- This fixes the issue where testing WGC with secondary monitor would show borders on both displays

Previously, calling engine.initialize() would initialize WGC's monitor 0 by default, then capture_display() would initialize the requested monitor, causing both to be active. Now all engines consistently auto-initialize only when needed.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

server/src/wled_controller/api/routes.py

@@ -930,9 +930,8 @@ async def test_template(
                 )
             )
 
-        # Create and initialize engine
+        # Create engine (auto-initializes on first capture)
         engine = EngineRegistry.create_engine(test_request.engine_type, test_request.engine_config)
-        engine.initialize()
 
         # Run sustained capture test
         logger.info(f"Starting {test_request.capture_duration}s capture test with {test_request.engine_type}")