fix(config): validate prediction providers before the prediction exists

The provider settings (weather, PV forecast, load, electricity price, feed-in
tariff, electricity fee) validate their provider id against the providers of
the prediction instance. While the configuration is created, that instance
does not exist yet, and the validation falls back to fixed lists. These were
incomplete: weather knew only 'WeatherImport', load lacked
'LoadAkkudoktorAdjusted'. A configuration file with e.g. weather provider
'OpenMeteo' then fails to load:

  Provider 'OpenMeteo' is not a valid weather provider: ['WeatherImport']

This is hidden today because ConfigEOS.documentation_mode() is always true
(_force_documentation_mode is a pydantic private attribute, whose class
attribute is a truthy descriptor), and get_prediction() then creates the
prediction instance on access. Once documentation mode is enabled only
explicitly, the configuration does not load.

Derive the provider ids from the provider classes instead
(prediction_provider_ids). They no longer depend on whether the prediction
instance exists, so the validation is the same before and after
initialization.

Additional changes
- Add the test fixture uncreated_config, which lets get_config(init=True)
  create a new configuration from the config file and restores the old one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/akkudoktoreos/prediction/weather.py

@@ -5,25 +5,17 @@ from typing import Optional
 from pydantic import Field, computed_field, field_validator
 
 from akkudoktoreos.config.configabc import SettingsBaseModel
-from akkudoktoreos.core.coreabc import get_prediction
 from akkudoktoreos.prediction.weatherabc import WeatherProvider
 from akkudoktoreos.prediction.weatherimport import WeatherImportCommonSettings
 
 
 def weather_provider_ids() -> list[str]:
     """Valid weather provider ids."""
-    try:
-        prediction_eos = get_prediction()
-    except Exception:
-        # Prediction may not be initialized
-        # Return at least provider used in example
-        return ["WeatherImport"]
+    # From the provider classes: valid also while the configuration is created,
+    # before the prediction instance exists.
+    from akkudoktoreos.prediction.prediction import prediction_provider_ids
 
-    return [
-        provider.provider_id()
-        for provider in prediction_eos.providers
-        if isinstance(provider, WeatherProvider)
-    ]
+    return prediction_provider_ids(WeatherProvider)
 
 
 class WeatherCommonSettings(SettingsBaseModel):