Refine instance supplier check in BeanDefinitionMethodGenerator

This commit refines the instance supplier check in
BeanDefinitionMethodGenerator constructor in order to allow
overriding by an AOT contribution.

Closes gh-29556

spring-beans/src/main/java/org/springframework/beans/factory/aot/BeanDefinitionMethodGenerator.java

@@ -78,7 +78,7 @@ class BeanDefinitionMethodGenerator {
 			List<BeanRegistrationAotContribution> aotContributions) {
 
 		RootBeanDefinition mbd = registeredBean.getMergedBeanDefinition();
-		if (mbd.getInstanceSupplier() != null) {
+		if (mbd.getInstanceSupplier() != null && aotContributions.isEmpty()) {
 			throw new IllegalArgumentException("Code generation is not supported for bean definitions declaring an instance supplier callback : " + mbd);
 		}
 		this.methodGeneratorFactory = methodGeneratorFactory;

spring-beans/src/test/java/org/springframework/beans/factory/aot/BeanDefinitionMethodGeneratorTests.java

@@ -105,6 +105,23 @@ class BeanDefinitionMethodGeneratorTests {
 		});
 	}
 
+	@Test // gh-29556
+	void generateBeanDefinitionMethodGeneratesMethodWithInstanceSupplier() {
+		RegisteredBean registeredBean = registerBean(new RootBeanDefinition(TestBean.class, TestBean::new));
+		BeanDefinitionMethodGenerator generator = new BeanDefinitionMethodGenerator(
+				this.methodGeneratorFactory, registeredBean, null,
+				List.of((generationContext, beanRegistrationCode) -> { }));
+		MethodReference method = generator.generateBeanDefinitionMethod(
+				this.generationContext, this.beanRegistrationsCode);
+		compile(method, (actual, compiled) -> {
+			SourceFile sourceFile = compiled.getSourceFile(".*BeanDefinitions");
+			assertThat(sourceFile).contains("Get the bean definition for 'testBean'");
+			assertThat(sourceFile).contains("beanType = TestBean.class");
+			assertThat(sourceFile).contains("setInstanceSupplier(TestBean::new)");
+			assertThat(actual).isInstanceOf(RootBeanDefinition.class);
+		});
+	}
+
 	@Test
 	void generateBeanDefinitionMethodWhenHasInnerClassTargetMethodGeneratesMethod() {
 		this.beanFactory.registerBeanDefinition("testBeanConfiguration", new RootBeanDefinition(
@@ -493,8 +510,8 @@ class BeanDefinitionMethodGeneratorTests {
 		testBeanDefinitionMethodInCurrentFile(Boolean.class, beanDefinition);
 	}
 
-	@Test
-	void throwExceptionWithInstanceSupplier() {
+	@Test // gh-29556
+	void throwExceptionWithInstanceSupplierWithoutAotContribution() {
 		RegisteredBean registeredBean = registerBean(new RootBeanDefinition(TestBean.class, TestBean::new));
 		assertThatIllegalArgumentException().isThrownBy(() -> new BeanDefinitionMethodGenerator(
 				this.methodGeneratorFactory, registeredBean, null,