HT: Introduce mass balance equation type and refactor thermal expansion term

In the current HT implementation, the flow equation is based on the volume‑balance formulation, which reduces solution accuracy when liquid density depends on either pressure or temperature or both. This MR introduces functionality that makes the mass‑balance equation mandatory whenever a non‑constant liquid density is used.

The thermal expansion part currently uses the Parameter based thermal expansivity and Biot’s coefficient, and it is never tested. This MR improves the computation for the monolithic scheme, replaces these parameters with the MPL ones, and updates two benchmarks including thermal expansion. With these changes, the input parameter <solid_thermal_expansion> is removed. Thermal expansion is now computed automatically if <thermal_expansivity> is defined in the solid phase and <biot_coefficient> is defined in the medium properties.

1. Tests covering your feature were added?
2. Any new feature or behaviour change was documented?

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