Add typed value getters to States container (get_value_as_*/first_value_as_*)

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## Summary

Complete the v2 `States` accessor ergonomics pass (started in #2108) by adding **typed value getters** to the `States` container, before the 2.0 freeze.

`States.get_value()` / `first_value()` return the untyped union:

```python
StateType = str | int | float | bool | dict[str, Any] | list[Any] | None
```

The typed accessors (`value_as_float`, `value_as_int`, …) exist only on the `State` object. So every consumer that needs a concrete type is pushed into one of two verbose shapes:

1. **Cast the union:**
```python
cast(float, device.states.get_value(OverkizState.CORE_TARGET_DWH_TEMPERATURE))
```
2. **`get()` + None-guard + `value_as_*`:**
```python
state = device.states.get(name)
if state is None:
return None
position = state.value_as_int
```

In the Home Assistant `overkiz` integration's pyOverkiz 2.0 migration this accounts for **52 `cast(...)` call sites across 23 files** (~26 `float`, ~22 `str`, ~3 `dict`, ~1 `int`), plus the two-step guard pattern in cover position fallbacks and every water-heater / Hitachi temperature property.

## Proposed addition

Thin pass-throughs delegating to the **already-existing, already-tested** `State.value_as_*` properties:

```python
# on States (backs both device.states and device.attributes)
def get_value_as_float(self, name: StateName) -> float | None:
state = self._index.get(name)
return state.value_as_float if state is not None else None

# ... get_value_as_int / _str / _bool / _dict / _list
# ... first_value_as_float(names) / ... for fallback chains
```

Usage collapses to:

```python
temp = device.states.get_value_as_float(OverkizState.CORE_TARGET_DWH_TEMPERATURE)
position = device.states.get_value_as_int(state_name) # None if missing
```

## Why before the 2.0 freeze

The addition is **non-breaking**, but it is freeze-sensitive:

- The new public **method names are locked at freeze** — renaming them post-2.0 would itself be a breaking change.
- Consumers that pin an exact version (e.g. Home Assistant) need the methods in the `2.0.0` final to adopt them in a **single migration pass** rather than re-touching every file again in 2.1.

## Design decisions to lock

- **Naming:** `get_value_as_float` (mirrors `State.value_as_float`) vs a shorter `get_float`. Whatever is chosen is frozen.
- **Fallback variant:** include `first_value_as_*` — serves fallback chains (e.g. cover "My position" / "Unknown position", towel-dryer setpoint selection).
- **Mismatch semantics:** `State.value_as_*` already raises `TypeError` on a genuine type mismatch (fail-fast). Keep that propagating; only a *missing* state returns `None`.

## Scope

- `pyoverkiz/models.py`: add the getters to `States` + the `first_value_as_*` variants.
- `tests/test_models.py`: cover hit / miss / type-mismatch / enum-key cases.
- `docs/device-control.md`: document the new getters alongside `get_value`.

Follow-up (separate, in home-assistant-core): adopt the getters and drop the 52 casts.

Relates to #2108.

Beginne in pyoverkiz/models.py damit, die vorhandenen States-get_value/first_value-Methoden und die State.value_as_*-Properties zu lesen. Füge die im Issue beschriebenen typisierten Getter- und Fallback-Varianten hinzu und erweitere anschließend tests/test_models.py um Treffer, Fehlschläge, Typkonflikte und enum-Schlüssel. Aktualisiere docs/device-control.md zusammen mit get_value; fertig ist die Aufgabe, wenn die Tests bestehen und die neuen Accessors dokumentiert sind.

Vom Indexierungsmodell aus dem Issue-Text verfasst.