fix(number): 🩹 correct off-by-one in dhw manual frequency mapping

[rhammen]

🐛 Bug

The number.luxtronik_..._dhw_manual_frequency entity displayed/wrote values 1 Hz higher than the heat pump's real manual DHW frequency (reported in #674).
E.g. the heat pump GUI shows 54 Hz while the HA entity showed 55 Hz.

Root cause: da00b30 changed the FrequencyAutomatic raw-to-Hz conversion from +19/-19 (raw 1-101 → 20-120 Hz) to +20/-20 (raw 1-101 → 21-121 Hz), and bumped native_max_value to 121 to match — but the heat pump's real range is 20-120 Hz.

✅ Fix

• lux_overrides.py: revert FrequencyAutomatic.from_heatpump/to_heatpump to the correct 20-120 Hz mapping
• number_entities_predefined.py: revert native_max_value from 121.0 to 120.0
• number.py: revert the async_set_native_value validation threshold from 0 < value < 21 to 0 < value < 20 so 20 Hz is a valid manual value again
• Updated affected unit tests in test_lux_overrides.py and test_number.py to assert the corrected 20-120 Hz range

⚠️ Known caveat

Per discussion on #674, the +20/121 mapping this PR reverts was originally introduced because of a separate observation: when the heat pump was set to a fixed DHW frequency of 30 Hz via its own controller, sensor.luxtronik2_pump_frequency in HA continuously reported 29 Hz — i.e. 1 Hz below the value actually configured on the unit.

This PR restores the entity's display/write range to match the heat pump's documented 20-120 Hz UI range (fixing the reported bug), but as a result the actual pump frequency may run 1 Hz lower than the value set via dhw_manual_frequency. That discrepancy appears to be a separate, pre-existing quirk in how the heat pump reports pump_frequency relative to a fixed DHW setpoint, not something this mapping fix can address without reintroducing the original off-by-one display bug. Tracking/investigating that separately may be worthwhile, but is out of scope here.

Test plan

• pytest — 747 passed
• ruff check / ruff format --check — clean
• basedpyright — 0 errors

Resolves #674

[github-actions]

Package	Line Rate	Health
custom_components.luxtronik2	100%	✔
Summary	100% (2873 / 2874)	✔