Annotate generated classes with @Generated

This commit annotates every generated class with `@Generated` so that
build tools can recognize and ignore those types if necessary.

Closes gh-30824

spring-core/src/test/java/org/springframework/aot/generate/GeneratedClassTests.java

@@ -94,6 +94,14 @@ class GeneratedClassTests {
 		assertThat(innerGeneratedClass).isSameAs(innerGeneratedClass2).isSameAs(innerGeneratedClass3);
 	}
 
+	@Test
+	void generateJavaFileIsAnnotatedWithGenerated() {
+		GeneratedClass generatedClass = createGeneratedClass(TEST_CLASS_NAME);
+		assertThat(generatedClass.generateJavaFile().toString())
+				.contains("@Generated")
+				.contains("import " + Generated.class.getName() + ";");
+	}
+
 	@Test
 	void generateJavaFileIncludesGeneratedMethods() {
 		GeneratedClass generatedClass = createGeneratedClass(TEST_CLASS_NAME);