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

Author: sbrannen

spring-core/src/main/java/org/springframework/aot/hint/ResourcePatternHint.java

@@ -43,7 +43,8 @@
  *         and its child directories at any depth.</li>
  * </ul>
  *
- * <p>A resource pattern must not start with a slash ({@code /}).
+ * <p>A resource pattern must not start with a slash ({@code /}) unless it is the
+ * root directory.
  *
  * @author Stephane Nicoll
  * @author Brian Clozel
@@ -58,13 +59,16 @@ public final class ResourcePatternHint implements ConditionalHint {
 	@Nullable
 	private final TypeReference reachableType;
 
+
 	ResourcePatternHint(String pattern, @Nullable TypeReference reachableType) {
-		Assert.isTrue(!pattern.startsWith("/"),
-				() -> "Resource pattern [%s] must not start with a '/'".formatted(pattern));
+		Assert.isTrue(("/".equals(pattern) || !pattern.startsWith("/")),
+				() -> "Resource pattern [%s] must not start with a '/' unless it is the root directory"
+						.formatted(pattern));
 		this.pattern = pattern;
 		this.reachableType = reachableType;
 	}
 
+
 	/**
 	 * Return the pattern to use for identifying the resources to match.
 	 * @return the pattern

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

@@ -81,6 +81,13 @@ void registerExactMatch() {
 				.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