Show action error for device assistant action when no assistant is installed

[keymapper-ai]

Closes #2091

Summary

• Added error checking for the "Open device assistant" action in ActionErrorSnapshot.kt: when no device assistant is configured on the device, the action now shows a KMError.NoDeviceAssistant error (using the existing getDeviceAssistantPackage() method to detect this state).
• Added two unit tests in GetActionErrorUseCaseTest covering both the error case (no assistant installed) and the happy path (assistant installed).

The KMError.NoDeviceAssistant error type, its display string (error_no_device_assistant), and the ErrorUtils.kt handler were all already present — only the snapshot check was missing, consistent with how ActionData.VoiceAssistant already handles KMError.NoVoiceAssistant.

Known limitations / areas for human review

• None — this is a small, targeted fix following the exact pattern used for the voice assistant action.

https://claude.ai/code/session_0116QYB2BM6u4VAhCRTcxZid