PageModelApiMount instances always return null for #getChannel

When this works fine in 13.2.0, we should even think about backporting this since in earlier versions this might be confusing. Also the commits/workaround in HSTTWO-4548 can then be undone (not the unit test, that one should keep working)

When an hst:mount defines the property

hst:pagemodelapi

then the value of this property (assume it is 'resourceapi') , then an in memory mount with name 'resourceapi' will be appended below the mount defining the property 'hst:pagemodelapi'. This way, the normal website can be accessed via the explicit mount, and the pagemodelapi output via the in memory added mount. However, the PageModelApiMount impl always returns null for #getChannel. This might result in problems if for example an HstComponent accessed #getChannel and expects it not to be null or wants to serialize its value via the pagemodelapi output. Currently, the HstComponent should add a check something like

 Mount mount = requestContext.getResolvedMount().getMount();
        if (PAGE_MODEL_PIPELINE_NAME.equals(mount.getNamedPipeline()) && mount.getParent() != null) {
            mount = mount.getParent();
        }

This is error prone and shouldn't be needed. Instead, we page model api mount on #getChannel should just try to return the channel of the parent mount by default.

Getting the parent mount can then be removed from

• CmsComponentComponentWindowAttributeContributor
• CmsComponentWindowResponseAppender
• AbstractVirtualBeanPropertyWriter