Relax singleton enforcement for Bean Overrides in the TestContext framework

In gh-33602, we introduced strict singleton enforcement for bean
overrides -- for example, for @⁠MockitoBean, @⁠TestBean, etc. However,
the use of BeanFactory#isSingleton(beanName) can result in a
BeanCreationException for certain beans, such as a Spring Data JPA
FactoryBean for a JpaRepository.

In light of that, this commit relaxes the singleton enforcement in
BeanOverrideBeanFactoryPostProcessor by only checking the result of
BeanDefinition#isSingleton() for existing bean definitions.

This commit also updates the Javadoc and reference documentation to
reflect the status quo.

See gh-33602
Closes gh-33800

framework-docs/modules/ROOT/pages/testing/annotations/integration-spring/annotation-mockitobean.adoc

@@ -39,8 +39,17 @@ xref:testing/testcontext-framework/bean-overriding.adoc#testcontext-bean-overrid
 and the original instance is wrapped in a Mockito spy. This strategy requires that
 exactly one candidate bean exists.
 
-NOTE: Only _singleton_ beans can be overridden. Any attempt to override a non-singleton
-bean will result in an exception.
+[TIP]
+====
+Only _singleton_ beans can be overridden. Any attempt to override a non-singleton bean
+will result in an exception.
+
+When using `@MockitoBean` to mock a bean created by a `FactoryBean`, the `FactoryBean`
+will be replaced with a singleton mock of the type of object created by the `FactoryBean`.
+
+When using `@MockitoSpyBean` to create a spy for a `FactoryBean`, a spy will be created
+for the object created by the `FactoryBean`, not for the `FactoryBean` itself.
+====
 
 The following example shows how to use the default behavior of the `@MockitoBean` annotation:
 

framework-docs/modules/ROOT/pages/testing/annotations/integration-spring/annotation-testbean.adoc

@@ -82,7 +82,7 @@ Java::
 <2> The result of this static method will be used as the instance and injected into the field.
 ======
 
-[NOTE]
+[TIP]
 ====
 Spring searches for the factory method to invoke in the test class, in the test class
 hierarchy, and in the enclosing class hierarchy for a `@Nested` test class.
@@ -92,5 +92,12 @@ fully-qualified method name following the syntax `<fully-qualified class name>#<
 – for example, `methodName = "org.example.TestUtils#createCustomService"`.
 ====
 
-NOTE: Only _singleton_ beans can be overridden. Any attempt to override a non-singleton
-bean will result in an exception.
+[TIP]
+====
+Only _singleton_ beans can be overridden. Any attempt to override a non-singleton bean
+will result in an exception.
+
+When overriding a bean created by a `FactoryBean`, the `FactoryBean` will be replaced
+with a singleton bean corresponding to the value returned from the `@TestBean` factory
+method.
+====

framework-docs/modules/ROOT/pages/testing/testcontext-framework/bean-overriding.adoc

@@ -57,6 +57,19 @@ defined by the corresponding `BeanOverrideStrategy`:
 `WRAP`::
   Retrieves the original bean and wraps it.
 
+[TIP]
+====
+Only _singleton_ beans can be overridden. Any attempt to override a non-singleton bean
+will result in an exception.
+
+When replacing a bean created by a `FactoryBean`, the `FactoryBean` itself will be
+replaced with a singleton bean corresponding to bean override instance created by the
+applicable `BeanOverrideHandler`.
+
+When wrapping a bean created by a `FactoryBean`, the object created by the `FactoryBean`
+will be wrapped, not the `FactoryBean` itself.
+====
+
 [NOTE]
 ====
 In contrast to Spring's autowiring mechanism (for example, resolution of an `@Autowired`
@@ -71,6 +84,3 @@ Alternatively, the user can directly provide the bean name in the custom annotat
 `BeanOverrideProcessor` implementations may also internally compute a bean name based on
 a convention or some other method.
 ====
-
-NOTE: Only _singleton_ beans can be overridden. Any attempt to override a non-singleton
-bean will result in an exception.