Expose a way to resolved a merged inner bean definition

This commit extracts the logic of resolving a merged bean definition for
an inner bean to a public method so that other components can reuse it.

Closes gh-28093

spring-beans/src/test/java/org/springframework/beans/factory/support/BeanDefinitionValueResolverTests.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2002-2022 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.beans.factory.support;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link BeanDefinitionValueResolver}.
+ *
+ * @author Stephane Nicoll
+ */
+class BeanDefinitionValueResolverTests {
+
+	@Test
+	void resolveInnerBean() {
+		DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
+		RootBeanDefinition parentBd = new RootBeanDefinition();
+		GenericBeanDefinition innerBd = new GenericBeanDefinition();
+		innerBd.setAttribute("test", 42);
+		BeanDefinitionValueResolver bdvr = new BeanDefinitionValueResolver(beanFactory, "test", parentBd);
+		RootBeanDefinition resolvedInnerBd = bdvr.resolveInnerBean(null,innerBd, (name, mbd) -> {
+			assertThat(name).isNotNull().startsWith("(inner bean");
+			return mbd;
+		});
+		assertThat(resolvedInnerBd.getAttribute("test")).isEqualTo(42);
+	}
+
+}