Use Attribute enum in remote

[epenet]

Proposed change

As follow-up to #174625

Linked to home-assistant/epics#104

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[home-assistant]

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[Copilot]

Pull request overview

This PR is a follow-up to #174625, part of epic #104 to migrate platform entity attributes to StrEnum. It updates the remote integration's significant-change helper to reference the new RemoteEntityStateAttribute enum instead of the legacy ATTR_CURRENT_ACTIVITY string constant, aligning it with the pattern already used by integrations like light and fan.

Changes:

• Import RemoteEntityStateAttribute from .const instead of ATTR_CURRENT_ACTIVITY from the package __init__.
• Use RemoteEntityStateAttribute.CURRENT_ACTIVITY for the significant-change attribute comparison.