Four test assertions carried over from the C# suite are still commented out

Four assertions carried over from the original C# test suite are still commented out:

- `tests/test_dfsu2D.py:404` — `# TODO: Assert.AreEqual(eumUnit.eumUmeter, axis.AxisUnit);`
- `tests/test_dfsu2D.py:469` — same
- `tests/test_dfsu2D.py:806` — `# TODO: Implement?`
- `tests/test_dfs_static_item.py:113` — `# TODO: Assert.AreEqual(eumUnit.eumUmeter, axis.AxisUnit`

The three `AxisUnit` ones are a single, small check in Python:

```python
assert axis.AxisUnit == eumUnit.eumUmeter
```

If that holds, the tests get slightly stronger for free. If it does not hold, that is more interesting than the test, and worth a separate issue.

`test_dfsu2D.py:806` is vaguer and may just be a note that the C# original tested something the Python port does not cover; it should either become a real test or be removed.

Small, self-contained, and a reasonable first contribution.

Beginne bei tests/test_dfsu2D.py in den Zeilen 404, 469 und 806 sowie bei tests/test_dfs_static_item.py in Zeile 113. Füge die drei angegebenen AxisUnit-Assertions hinzu und ermittle dann, ob das verbleibende TODO zu einem echten Test werden oder entfernt werden sollte. Führe die betroffenen Testdateien aus und bestätige, dass die Assertions erfolgreich sind und kein auskommentiertes TODO verbleibt.

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