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_elecpricefixed.py

@@ -1,12 +1,8 @@
 """Tests for fixed electricity price prediction module."""
 
-import asyncio
 import json
 from pathlib import Path
-from unittest.mock import Mock, patch
 
-import numpy as np
-import pandas as pd
 import pytest
 
 from akkudoktoreos.config.configabc import ValueTimeWindow, ValueTimeWindowSequence
@@ -16,7 +12,7 @@ from akkudoktoreos.prediction.elecpricefixed import (
     ElecPriceFixed,
     ElecPriceFixedCommonSettings,
 )
-from akkudoktoreos.utils.datetimeutil import Duration, to_datetime
+from akkudoktoreos.utils.datetimeutil import to_datetime
 
 DIR_TESTDATA = Path(__file__).absolute().parent.joinpath("testdata")
 FILE_TESTDATA_ELECPRICEFIXED_CONFIG_JSON = DIR_TESTDATA.joinpath("elecpricefixed_config.json")