fix: wire up nightlight brightness/toggle for Sprout Humidifier (LEH-B381S)

[rakeshpyboina]

Summary

The LEH-B381S (Sprout Humidifier) already declares NIGHTLIGHT / NIGHTLIGHT_BRIGHTNESS support in device_map.py, and VeSyncSproutHumid already had a working private _set_nightlight_state helper that builds the correct setLightStatus payload. However, the class never overrode the base class's set_nightlight_brightness / toggle_nightlight methods, so any caller (e.g. Home Assistant's select/switch entities) fell through to the base-class stub, which just logs Nightlight brightness has not been configured. and returns False instead of calling the API.

Traceback this was pulled from (Home Assistant, via select.async_select_option):

File ".../vesync/select.py", line 194, in async_select_option
ERROR [pyvesync.base_devices.humidifier_base] Nightlight brightness has not been configured.

• Added set_nightlight_brightness and toggle_nightlight overrides to VeSyncSproutHumid, wired to the existing _set_nightlight_state helper (mirroring the pattern used in VeSyncHumid200300S/VeSyncHumid1000S).
• Fixed a latent bug in _set_nightlight_state: it unconditionally stored the passed-in brightness (including None) as the new state, and would send "brightness": null to the API when toggling the light before a brightness had ever been read or set. It now falls back to the last known brightness, or 100 if none is known yet.

Test plan

• Added turn_on_nightlight, turn_off_nightlight, and set_nightlight_brightness test cases for LEH-B381S in test_humidifiers.py
• Recorded new API fixtures in src/tests/api/vesynchumidifier/LEH-B381S.yaml
• pytest src/tests/ — 355 passed, 3 skipped
• mypy src/pyvesync/devices/vesynchumidifier.py — no issues
• Validated against a physical LEH-B381S device (reporter has the device; will confirm in this thread)

🤖 Generated with Claude Code

[rakeshpyboina]

Validated against my physical LEH-B381S (Sprout Humidifier) and VeSync account. All three methods work correctly end-to-end:

```
Before: nightlight_status=on, nightlight_brightness=51

--- turn_on_nightlight() ---
success=True, status=on, brightness=51

--- set_nightlight_brightness(30) ---
success=True, status=on, brightness=30

--- turn_off_nightlight() ---
success=True, status=off, brightness=30
```

Confirmed each state change matched what happened on the physical device/Levoit app.