Support displaying custom actions on semantic group containers (accessibilityContainerType)

[abey]

When a view is configured as an accessibility container using accessibilityContainerType = .semanticGroup and has accessibilityCustomActions set, the current snapshot rendering does not display the "Actions Available" indicator or list the custom actions for that container.

In VoiceOver, semantic group containers can have custom actions that users can access via the Actions rotor. When focused on any child element within the semantic group, VoiceOver announces "actions available" and allows users to swipe up/down to access the actions.

Currently:

• Custom actions on individual accessibility elements (isAccessibilityElement = true) - Shows "Actions Available" with action list
• Custom actions on semantic group containers (accessibilityContainerType = .semanticGroup) - Does NOT show actions

Ideally, both cases should display the "Actions Available" indicator and list the custom actions, since both are valid VoiceOver-accessible patterns.

Example Code:

// works
let imageView = UIImageView()
imageView.isAccessibilityElement = true
imageView.accessibilityCustomActions = [UIAccessibilityCustomAction(name: "foo", ...)]

// doesn't work
let containerView = UIView()
containerView.accessibilityContainerType = .semanticGroup
containerView.accessibilityCustomActions = [
    UIAccessibilityCustomAction(name: "foo", ...),
    UIAccessibilityCustomAction(name: "bar", ...),
    UIAccessibilityCustomAction(name: "baz", ...),
]