Support Bean Overrides for non-singletons

Prior to this commit, the BeanOverrideBeanFactoryPostProcessor rejected
any attempt to override a non-singleton bean; however, due to interest
from the community, we have decided to provide support for overriding
non-singleton beans via the Bean Override mechanism — for example, when
using @⁠MockitoBean, @⁠MockitoSpyBean, and @⁠TestBean.

With this commit, we now support Bean Overrides for non-singletons: for
standard JVM runtimes as well as AOT processing and AOT runtimes. This
commit also documents that non-singletons will effectively be converted
to singletons when overridden and logs a warning similar to the
following.

WARN: BeanOverrideBeanFactoryPostProcessor - Converting 'prototype' scoped bean definition 'myBean' to a singleton.

See gh-33602
See gh-32933
See gh-33800
Closes gh-35574

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

@@ -88,14 +88,20 @@ To avoid such undesired side effects, consider using
 
 [NOTE]
 ====
-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 non-singleton bean, the non-singleton bean will be
+replaced with a singleton mock, and the corresponding bean definition will be converted
+to a `singleton`. Consequently, if you mock a `prototype` or scoped bean, the mock will
+be treated as a `singleton`.
+
+Similarly, when using `@MockitoSpyBean` to create a spy for a non-singleton bean, the
+corresponding bean definition will be converted to a `singleton`. Consequently, if you
+create a spy for a `prototype` or scoped bean, the spy will be treated as a `singleton`.
 
 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.
+Similarly, 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.
 ====
 
 [NOTE]

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

@@ -116,12 +116,15 @@ fully-qualified method name following the syntax `<fully-qualified class name>#<
 – for example, `methodName = "org.example.TestUtils#createCustomService"`.
 ====
 
-[TIP]
+[NOTE]
 ====
-Only _singleton_ beans can be overridden. Any attempt to override a non-singleton bean
-will result in an exception.
+When overriding a non-singleton bean, the non-singleton bean will be replaced with a
+singleton bean corresponding to the value returned from the `@TestBean` factory method,
+and the corresponding bean definition will be converted to a `singleton`. Consequently,
+if `@TestBean` is used to override a `prototype` or scoped bean, the overridden bean will
+be treated as a `singleton`.
 
-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.
+Similarly, 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

@@ -62,8 +62,11 @@ defined by the corresponding `BeanOverrideStrategy`:
 
 [TIP]
 ====
-Only _singleton_ beans can be overridden. Any attempt to override a non-singleton bean
-will result in an exception.
+When replacing a non-singleton bean, the non-singleton bean will be replaced with a
+singleton bean corresponding to bean override instance created by the applicable
+`BeanOverrideHandler`, and the corresponding bean definition will be converted to a
+`singleton`. Consequently, if a handler overrides a `prototype` or scoped bean, the
+overridden bean will be treated as a `singleton`.
 
 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