convert to integer if rounding precision is zero (#30226)

Convert values to integer if rounding precision is zero. With that a
value which is an integer before filtering can be configured to stay
integer when using precision = 0. This also aligns behavior of filters
to how rounding behaves in tempaltes (homeassistant/helpers/template.py,
function forgiving_round).

homeassistant/components/filter/sensor.py

@@ -324,7 +324,8 @@ class FilterState:
     def set_precision(self, precision):
         """Set precision of Number based states."""
         if isinstance(self.state, Number):
-            self.state = round(float(self.state), precision)
+            value = round(float(self.state), precision)
+            self.state = int(value) if precision == 0 else value
 
     def __str__(self):
         """Return state as the string representation of FilterState."""

tests/components/filter/test_sensor.py

@@ -208,6 +208,13 @@ class TestFilterSensor(unittest.TestCase):
             filtered = filt.filter_state(state)
         assert 21 == filtered.state
 
+    def test_precision_zero(self):
+        """Test if precision of zero returns an integer."""
+        filt = LowPassFilter(window_size=10, precision=0, entity=None, time_constant=10)
+        for state in self.values:
+            filtered = filt.filter_state(state)
+        assert isinstance(filtered.state, int)
+
     def test_lowpass(self):
         """Test if lowpass filter works."""
         filt = LowPassFilter(window_size=10, precision=2, entity=None, time_constant=10)