Fix light: restore last brightness on turn-on instead of staying dark

[danielbrowne]

Problem

The light's on/off toggle is effectively one-way: you can turn the light off, but a plain toggle won't turn it back on. From the UI (light card tap, or the more-info dialog toggle) the light goes dark and then won't come back — and cycling the power switch doesn't help either. The only way to recover is to set an explicit brightness/color (e.g. from the Hatch app).

Root cause

The device has no separate lamp power, so the integration models light "off" as brightness = 0. But async_turn_on only calls set_brightness/set_color when an explicit value is passed in:

if brightness:
    await self._hatch_rest_device.set_brightness(brightness)
if rgb:
    await self._hatch_rest_device.set_color(*rgb)

A bare turn_on() (no attributes) therefore powers the device on but leaves brightness at 0, so is_on stays False and the light appears stuck off. The power switch only toggles device power (SI), not brightness, so it can't rescue it.

Fix

• Remember the brightness in async_turn_off before zeroing it.
• In async_turn_on, when no brightness is supplied and the light is currently at 0, restore the remembered brightness (falling back to full brightness when unknown).

Behaviour with an explicit brightness or color is unchanged; a bare toggle now round-trips off→on like a normal dimmable light. No new dependencies.

Tests

Adds coverage to tests/test_light.py:

• test_async_turn_off_remembers_brightness
• test_async_turn_on_restores_last_brightness
• test_async_turn_on_defaults_full_when_unknown
• test_async_turn_on_no_restore_when_already_lit (doesn't clobber current brightness)
• test_turn_off_then_on_round_trips_brightness

All existing light tests remain valid (the restore path only triggers when current brightness is 0).

Notes

Verified against a real 1st-gen Hatch Rest.