Add missing @Nullable type annotations to getBean signatures

[subhramit]

Context

Nullaway had some warnings highlighted in https://github.com/uber/NullAway/actions/runs/27324825986/job/80868514555
On taking a look, they seemed to have merit.

For example, in

spring-boot/core/spring-boot-test/src/main/java/org/springframework/boot/test/context/assertj/ApplicationContextAssert.java

Lines 365 to 378 in 8ccf2e9

	public <T> AbstractObjectAssert<?, T> getBean(String name, Class<T> type) {
	if (this.startupFailure != null) {
	throwAssertionError(contextFailedToStartWhenExpecting(this.startupFailure,
	"to contain a bean of name:%n <%s> (%s)", name, type));
	}
	Object bean = findBean(name);
	if (bean != null && type != null && !type.isInstance(bean)) {
	throwAssertionError(new BasicErrorMessageFactory(
	"%nExpecting:%n <%s>%nto contain a bean of name:%n <%s> (%s)%nbut found:%n <%s> of type <%s>",
	getApplicationContext(), name, type, bean, bean.getClass()));
	}
	return assertThat((T) bean).as("Bean of name <%s> and type <%s> from <%s>", name, type,
	getApplicationContext());
	}

assertThat(bean) where bean is @Nullable T infers ObjectAssert<@Nullable T>, i.e. AbstractObjectAssert<ObjectAssert<@Nullable T>, @Nullable T>. The declared return type is AbstractObjectAssert<?, T> with a non-null T in the actual (second) type-argument. So @Nullable T flowing into a T slot is flagged by NullAway.

The javadoc also states:

spring-boot/core/spring-boot-test/src/main/java/org/springframework/boot/test/context/assertj/ApplicationContextAssert.java

Lines 348 to 349 in 8ccf2e9

	* bean becoming the object under test. If no bean of the specified name can be found
	* an assert on {@code null} is returned.

So the actual value under test genuinely is nullable.

The PR fixes this and two other cases by making the return types of covered getBean signatures honest with @Nullable, thus also bringing the signatures in line with the documented behavior.

[subhramit]

Closing as covered by #50701.