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
81d89f478a