The problem

Hello.
Absolute humidity sensors with "device_class":"humidity" and "unit_of_meas":"g/m³" fail because the unit of measure g/m³ is considered invalid for "device_class":"humidity".
I think this is a mistake, because absolute humidity is one of the main ways to measure air humidity, and I also think it is appropriate to keep the device_class humidity for absolute humidity measurement.
So I propose to add the unit of measure g/m³ to the device_class humidity.

What version of Home Assistant Core has the issue?

2025.7.1

What was the last working version of Home Assistant Core?

No response

What type of installation are you running?

Home Assistant OS

Integration causing the issue

MQTT

Link to integration documentation on our website

https://www.home-assistant.io/integrations/mqtt

Diagnostics information

No response

Example YAML snippet

Anything in the logs that might be useful for us?

2025-07-05 08:24:55.598 ERROR (MainThread) [homeassistant.components.mqtt.entity] Error 'The unit of measurement `g/m³` is not valid together with device class `humidity`' when processing MQTT discovery message topic: 'homeassistant/sensor/0280E10000F6-hum_abs/config', message: '{'state_class': 'measurement', 'device': {'identifiers': ['0280E10000F6'], 'connections': [['mac', '0280E10000F6']], 'manufacturer': 'ALGOL', 'model': 'Sensus191', 'via_device': 'OpenMQTTGateway_ESP32_OLM_POE', 'name': 'PEMS'}, 'state_topic': '+/+/BTtoMQTT/0280E10000F6', 'value_template': '{{value_json.hum_abs|is_defined}}', 'unique_id': '0280E10000F6-hum_abs', 'unit_of_measurement': 'g/m³', 'name': 'Sensus191-hum_abs', 'device_class': 'humidity'}'

Additional information

No response