TST: improve metric_prefix coverage

Author: luxedo

tests/test_protocol.py

@@ -209,6 +209,7 @@ def test_parse_success(self, unit, value):
             "abc",
             "petermeters",
             "nanopeters",
+            "Xm",
         ],
     )
     def test_parse_fail(self, test_value):
@@ -261,6 +262,10 @@ def test_from_float(self, test_value, answer_mantissa, answer_exponent):
         assert pytest.approx(val) == answer_mantissa
         assert mprefix is answer_exponent
 
+    def test_from_float_raises(self):
+        with pytest.raises(ValueError, match="Value is out of range"):
+            MetricPrefix.from_float(1e123)
+
     @pytest.mark.parametrize(
         ("test_value", "test_exponent", "answer_mantissa", "answer_exponent"),
         [