Check type and range of REFRESH-INTERVAL in setter

See https://github.com/collective/icalendar/pull/877#discussion_r2366353723

src/icalendar/cal/calendar.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+from datetime import timedelta
 from typing import TYPE_CHECKING, Sequence
 
 from icalendar.attr import (
@@ -21,7 +22,7 @@ from icalendar.version import __version__
 
 if TYPE_CHECKING:
     import uuid
-    from datetime import date, datetime, timedelta
+    from datetime import date, datetime
 
     from icalendar.cal.availability import Availability
     from icalendar.cal.event import Event
@@ -480,6 +481,9 @@ Description:
             the calendar data.  The value of this property SHOULD be used by
             calendar user agents to limit the polling interval for calendar
             data updates to the minimum interval specified.
+
+        Raises:
+            ValueError: When setting a negative duration.
         """
         refresh_interval = self.get("REFRESH-INTERVAL")
         return refresh_interval.dt if refresh_interval else None
@@ -487,6 +491,12 @@ Description:
     @refresh_interval.setter
     def refresh_interval(self, value: timedelta | None):
         """Set the REFRESH-INTERVAL."""
+        if not isinstance(value, timedelta) and value is not None:
+            raise TypeError(
+                "REFRESH-INTERVAL must be a positive timedelta or None (to delete it)."
+            )
+        if value is not None and value.total_seconds() < 0:
+            raise ValueError("REFRESH-INTERVAL must be a positive timedelta.")
         del self.refresh_interval
         if value is not None:
             self.add("REFRESH-INTERVAL", value)

src/icalendar/tests/test_rfc_7986.py

@@ -6,7 +6,7 @@ They are also considered.
 
 from __future__ import annotations
 
-from datetime import datetime, timedelta, timezone
+from datetime import date, datetime, time, timedelta, timezone
 from typing import Union
 
 import pytest
@@ -219,6 +219,34 @@ def test_refresh_interval_default(calendar: Calendar):
     assert calendar.refresh_interval is None
 
 
+@pytest.mark.parametrize(
+    "invalid_value",
+    [
+        datetime(2020, 1, 1),
+        date(2020, 1, 1),
+        "invalid",
+        (date(2020, 1, 1), timedelta(days=1)),
+        time(12, 0),
+    ],
+)
+def test_invalid_refresh_interval_type(calendar: Calendar, invalid_value):
+    """Invalid REFRESH-INTERVAL"""
+    with pytest.raises(TypeError):
+        calendar.refresh_interval = invalid_value
+
+
+def test_invalid_refresh_interval(calendar: Calendar):
+    with pytest.raises(ValueError):
+        calendar.refresh_interval = timedelta(seconds=-1)
+
+
+def test_0_refresh_interval(calendar: Calendar):
+    """REFRESH-INTERVAL zero."""
+    calendar.refresh_interval = timedelta(0)
+    assert calendar.refresh_interval == timedelta(0)
+    assert calendar["REFRESH-INTERVAL"].dt == timedelta(0)
+
+
 def test_refresh_interval_set_to_value(calendar: Calendar):
     """REFRESH-INTERVAL setting."""
     calendar.refresh_interval = timedelta(hours=1)