test: consolidate suites and remove redundant cases (#1459)

Regression tests were scattered across feature- and bug-specific files, and some suites repeated the same checks. Group tests by responsibility, isolate feature fixtures in test classes, and remove duplicate or obsolete cases.

The review covered all 127 collected test modules on main. This reduces them and removes or combines definitions:.

 No application code changes.

tests/test_pvforecastpvlib.py

@@ -1,12 +1,9 @@
 
 """Tests for PVForecastPVLib prediction provider."""
 
-import bz2
-import pickle
 from pathlib import Path
-from unittest.mock import MagicMock, Mock, patch
+from unittest.mock import MagicMock, patch
 
-import numpy as np
 import pandas as pd
 import pytest
 
@@ -1054,7 +1051,6 @@ class TestProvider:
         weather = pd.DataFrame()
 
 
-
         with patch.object(
             Prediction,
             "keys_to_dataframe",
@@ -1285,28 +1281,6 @@ class TestProvider:
 class TestCalculation:
     """Tests for PVLib power calculation."""
 
-    @pytest.mark.skip("PVLib bails out on empty weather data")
-    def test_calculate_empty_weather(self, provider, sample_settings_4planes):
-        """Empty weather dataframe returns empty result."""
-
-        weather = pd.DataFrame(
-            columns=[
-                "ghi",
-                "dni",
-                "dhi",
-                "temp_air",
-                "wind_speed",
-                "relative_humidity",
-                "precipitable_water",
-                "cloud_cover",
-            ],
-            index=pd.DatetimeIndex([], tz="Europe/Berlin"),
-        )
-
-        result = provider._calculate_pvlib_power(weather)
-
-        assert isinstance(result, pd.DataFrame)
-        assert result.empty
 
     @patch("pvlib.modelchain.ModelChain")
     @patch("pvlib.location.Location")