Keep last seen sensor available after first value

[LordMike]

Why

This follows up on the concern raised in #31 (comment).

The last_seen sensor represents historical Bluetooth observation data, not the current live state of the display. If the device disappears for a while, the last-seen timestamp is still meaningful; making the entity unavailable effectively turns "last seen at X" into "unknown" at exactly the time the timestamp is most useful.

Other entities like temperature, RSSI, battery, button events, and touch events are live advertisement-derived state, so tying them to device availability makes sense. last_seen is different: once it has observed a timestamp, it should remain available and keep reporting that timestamp.

What Changed

• Changed only the last_seen sensor availability behavior.
• last_seen now caches the first observed timestamp.
• Before any timestamp has been observed, it still follows the coordinator availability.
• After a timestamp has been observed, it remains available even if the coordinator later marks the device unavailable.
• If Bluetooth history later returns no service info, the sensor keeps returning the cached last-seen timestamp.

Validation

• PYTHONDONTWRITEBYTECODE=1 PYTHONPATH=/mnt/n/Personal/opendisplay-work/Home_Assistant_Integration /home/mike/.venvs/hass-dev/bin/python -m pytest -q tests/test_last_seen.py -p no:cacheprovider
• Result: 4 passed