Use consistent exception messages in Assert calls

Update `Assert` calls to consistently use messages of the form
"'item' must [not] ...".

Closes gh-43780

spring-boot-project/spring-boot-testcontainers/src/test/java/org/springframework/boot/testcontainers/service/connection/FieldOriginTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2025 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.
@@ -36,7 +36,7 @@ class FieldOriginTests {
 	@Test
 	void createWhenFieldIsNullThrowsException() {
 		assertThatIllegalArgumentException().isThrownBy(() -> new FieldOrigin(null))
-			.withMessage("Field must not be null");
+			.withMessage("'field' must not be null");
 	}
 
 	@Test