Allow registration of resource hint for root directory

Prior to this commit, it was not possible to register the root
directory as a native image resource; however, it is necessary to be
able to register the root directory to enable classpath scanning of the
root directory within a native image -- for example, to support
resource patterns such as `classpath*:/*.properties`.

This commit therefore relaxes the precondition check in the
ResourcePatternHint constructor to allow explicit registration of the
root directory.

Closes gh-29402

spring-core/src/test/java/org/springframework/aot/hint/ResourceHintsTests.java

@@ -81,6 +81,13 @@ class ResourceHintsTests {
 				.hasSize(2);
 	}
 
+	@Test
+	void registerRootDirectory() {
+		this.resourceHints.registerPattern("/");
+		assertThat(this.resourceHints.resourcePatternHints()).singleElement().satisfies(
+				patternOf("/"));
+	}
+
 	@Test
 	void registerPattern() {
 		this.resourceHints.registerPattern("com/example/*.properties");

spring-core/src/test/java/org/springframework/aot/hint/ResourcePatternHintTests.java

@@ -33,7 +33,15 @@ class ResourcePatternHintTests {
 	void patternWithLeadingSlashIsRejected() {
 		assertThatIllegalArgumentException()
 				.isThrownBy(() -> new ResourcePatternHint("/file.properties", null))
-				.withMessage("Resource pattern [/file.properties] must not start with a '/'");
+				.withMessage("Resource pattern [/file.properties] must not start with a '/' unless it is the root directory");
+	}
+
+	@Test
+	void rootDirectory() {
+		ResourcePatternHint hint = new ResourcePatternHint("/", null);
+		assertThat(hint.toRegex().asMatchPredicate())
+				.accepts("/")
+				.rejects("/com/example", "/file.txt");
 	}
 
 	@Test