feat: add CakeDayConfig record constructor

Author: christolis

application/src/main/java/org/togetherjava/tjbot/config/CakeDayConfig.java

@@ -2,9 +2,18 @@
 
 import com.fasterxml.jackson.annotation.JsonProperty;
 
+import java.util.Objects;
+
 /**
  * Configuration record for the Cake Day feature.
  */
 public record CakeDayConfig(
         @JsonProperty(value = "rolePattern", required = true) String rolePattern) {
+
+    /**
+     * Configuration constructor for the Cake Day feature.
+     */
+    public CakeDayConfig {
+        Objects.requireNonNull(rolePattern);
+    }
 }