Refactor: Simplify test imports and improve documentation

- Replace convoluted importlib mechanics with standard import in
  test_soil_obs_conversion.py (reduces 14 lines to 3)
- Clarify df_state column count comment to explain the 28 soil
  observation fields (4 fields × 7 surfaces)
- Note that Water/Bldgs surfaces don't need soil obs but get
  columns anyway due to flat df_state structure

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: sunt05

test/cmd/test_df_state_conversion.py

@@ -94,7 +94,7 @@ def test_convert_old_csv(self, old_csv_path):
         assert len(df_new) == len(df_old)
         assert (
             len(df_new.columns) == 1451
-        )  # New format has 1451 columns (1420 + 3 view factors + 28 soil obs fields)
+        )  # New format: 1420 old + 3 view factors + 28 soil obs (4 fields × 7 surfaces, though Water/Bldgs don't need soil obs)
 
         # Check old columns removed
         col_names = {

test/core/test_soil_obs_conversion.py

@@ -1,18 +1,7 @@
-import sys
-from pathlib import Path
-import importlib.util
-
 import pandas as pd
 import pytest
 
-REPO_ROOT = Path(__file__).resolve().parents[2]
-MODULE_PATH = REPO_ROOT / "src" / "supy" / "util" / "_forcing.py"
-spec = importlib.util.spec_from_file_location("util._forcing", MODULE_PATH)
-soil_obs = importlib.util.module_from_spec(spec)
-assert spec.loader is not None
-sys.modules[spec.name] = soil_obs
-spec.loader.exec_module(soil_obs)
-convert_observed_soil_moisture = soil_obs.convert_observed_soil_moisture
+from supy.util._forcing import convert_observed_soil_moisture
 
 
 def _make_state(