Ignore non-String keys in PropertiesPropertySource.getPropertyNames()

Closes gh-32742

spring-core/src/main/java/org/springframework/core/env/PropertiesPropertySource.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -48,7 +48,7 @@ public class PropertiesPropertySource extends MapPropertySource {
 	@Override
 	public String[] getPropertyNames() {
 		synchronized (this.source) {
-			return super.getPropertyNames();
+			return ((Map<?, ?>) this.source).keySet().stream().filter(k -> k instanceof String).toArray(String[]::new);
 		}
 	}