Migrate PumpSteer sensors to SensorEntity and native_value/unit APIs

[JohanAlvedal]

Motivation

• Bring the PumpSteer sensor implementations in line with Home Assistant SensorEntity best practices by using native value/unit APIs and modern imports while keeping existing control logic intact.

Description

• Convert PumpSteerSensor and PumpSteerMLSensor to inherit from SensorEntity and update imports to use homeassistant.components.sensor.
• Replace legacy state/unit_of_measurement usage with native_value/native_unit_of_measurement and use SensorDeviceClass.TEMPERATURE and SensorStateClass.MEASUREMENT for the main sensor.
• Expose availability through _attr_available and set the entity icon via _attr_icon, while ensuring the attributes dict does not include friendly_name, unit_of_measurement, device_class, or icon keys; the fake_outdoor_temperature attribute is populated from the native value with one decimal place.
• No business logic or calculations were changed; only entity base classes, state/unit APIs, and related properties were adjusted.

Testing

• Ran pytest -q twice in the workspace; collection failed both times with ModuleNotFoundError: No module named 'homeassistant' due to the test environment not providing the Home Assistant package.

---

Codex Task