Add test for set_effect with const char* literal

Ensures the const char* overload resolves the ambiguous call
reported in #14728.

tests/components/light/common.yaml

@@ -60,6 +60,12 @@ esphome:
             }
           }
 
+      # Test set_effect with const char* doesn't cause ambiguous overload (issue #14728)
+      - lambda: |-
+          auto call = id(test_monochromatic_light).turn_on();
+          call.set_effect("None");
+          call.perform();
+
       - light.toggle: test_binary_light
       - light.turn_off: test_rgb_light
       - light.turn_on: