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>

tests/conftest.py

@@ -462,6 +462,31 @@ def config_eos(config_eos_factory) -> ConfigEOS:
     return config_eos
 
 
+@pytest.fixture
+def uncreated_config(config_eos: ConfigEOS, monkeypatch: pytest.MonkeyPatch) -> Path:
+    """Drop the created configuration, so get_config(init=True) creates a new one from the file.
+
+    Write the configuration to the returned config file path, then call
+    ``get_config(init=True)``. The new configuration reads the config file, as
+    the server does. monkeypatch puts the fixture's configuration back into
+    both ``coreabc._config_eos`` and the ``ConfigEOS`` singleton registry, so
+    the two stay in sync after the test.
+
+    Returns:
+        Path of the config file the new configuration reads.
+    """
+    from akkudoktoreos.core import coreabc
+
+    config_file_path = config_eos.general.config_file_path
+    if config_file_path is None:
+        raise RuntimeError("The test configuration has no config file path.")
+    singleton_instances: dict[type, object] = ConfigEOS._instances
+    monkeypatch.setattr(coreabc, "_config_eos", None)
+    monkeypatch.setattr(ConfigEOS, "_init_config_eos", {"with_file_settings": True})
+    monkeypatch.delitem(singleton_instances, ConfigEOS, raising=False)
+    return config_file_path
+
+
 # ------------------------------------
 # Provide pytest EOS server management
 # ------------------------------------