Rewrite Notification.ContentType as ContentType

[mokagio]

It was not clear whether the Notification it originally extended was the one from Foundation or from our model. But given the type is used internally and directly, that is without any reference to its container, we can simply make it a top level type and bypass the issue.

I haven't tested this on device.

Part of #24165

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 25.9. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

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