Support lower case configuration properties system environment names

Update `SystemEnvironmentPropertyMapper` to generate both mappings
in both the original case and upper case.

This restores the behavior of Spring Boot 3.4 where the system
environment could contain lowercase names.

Fixes gh-45741

Author: philwebb

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/source/ConfigurationPropertyName.java

@@ -543,25 +543,24 @@ ConfigurationPropertyName asSystemEnvironmentLegacyName() {
 
 	@Override
 	public String toString() {
-		return toString(ToStringFormat.DEFAULT);
+		return toString(ToStringFormat.DEFAULT, false);
 	}
 
-	String toString(ToStringFormat format) {
+	String toString(ToStringFormat format, boolean upperCase) {
 		String string = this.string[format.ordinal()];
 		if (string == null) {
 			string = buildToString(format);
 			this.string[format.ordinal()] = string;
 		}
-		return string;
+		return (!upperCase) ? string : string.toUpperCase(Locale.ENGLISH);
 	}
 
 	private String buildToString(ToStringFormat format) {
 		return switch (format) {
 			case DEFAULT -> buildDefaultToString();
-			case SYSTEM_ENVIRONMENT ->
-				buildSimpleToString('_', (i) -> getElement(i, Form.UNIFORM).toUpperCase(Locale.ENGLISH));
-			case LEGACY_SYSTEM_ENVIRONMENT -> buildSimpleToString('_',
-					(i) -> getElement(i, Form.ORIGINAL).replace('-', '_').toUpperCase(Locale.ENGLISH));
+			case SYSTEM_ENVIRONMENT -> buildSimpleToString('_', (i) -> getElement(i, Form.UNIFORM));
+			case LEGACY_SYSTEM_ENVIRONMENT ->
+				buildSimpleToString('_', (i) -> getElement(i, Form.ORIGINAL).replace('-', '_'));
 		};
 	}
 

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/source/SpringConfigurationPropertySource.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.context.properties.source;
 
+import java.util.Locale;
 import java.util.Map;
 import java.util.Random;
 
@@ -109,7 +110,8 @@ protected final Object getPropertySourceProperty(String name) {
 	}
 
 	Object getSystemEnvironmentProperty(Map<String, Object> systemEnvironment, String name) {
-		return systemEnvironment.get(name);
+		Object value = systemEnvironment.get(name);
+		return (value != null) ? value : systemEnvironment.get(name.toLowerCase(Locale.ROOT));
 	}
 
 	@Override

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/source/SystemEnvironmentPropertyMapper.java

@@ -16,8 +16,6 @@
 
 package org.springframework.boot.context.properties.source;
 
-import java.util.Arrays;
-import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
 import java.util.function.BiPredicate;
@@ -42,12 +40,10 @@ final class SystemEnvironmentPropertyMapper implements PropertyMapper {
 
 	@Override
 	public List<String> map(ConfigurationPropertyName configurationPropertyName) {
-		String name = configurationPropertyName.toString(ToStringFormat.SYSTEM_ENVIRONMENT);
-		String legacyName = configurationPropertyName.toString(ToStringFormat.LEGACY_SYSTEM_ENVIRONMENT);
-		if (name.equals(legacyName)) {
-			return Collections.singletonList(name);
-		}
-		return Arrays.asList(name, legacyName);
+		return List.of(configurationPropertyName.toString(ToStringFormat.SYSTEM_ENVIRONMENT, true),
+				configurationPropertyName.toString(ToStringFormat.LEGACY_SYSTEM_ENVIRONMENT, true),
+				configurationPropertyName.toString(ToStringFormat.SYSTEM_ENVIRONMENT, false),
+				configurationPropertyName.toString(ToStringFormat.LEGACY_SYSTEM_ENVIRONMENT, false));
 	}
 
 	@Override