feat(calendar): make external mutations durable

tests/test_capabilities.py

@@ -0,0 +1,27 @@
+from __future__ import annotations
+
+import unittest
+from datetime import datetime, timezone
+
+from govoplan_calendar.backend.capabilities import SqlCalendarSchedulingProvider
+from govoplan_core.core.calendar import CalendarCapabilityError, CalendarEventRequest
+
+
+class CalendarSchedulingCapabilityTests(unittest.TestCase):
+    def test_event_request_validation_is_exposed_as_capability_error(self) -> None:
+        provider = SqlCalendarSchedulingProvider()
+
+        with self.assertRaises(CalendarCapabilityError):
+            provider.create_event(
+                object(),
+                tenant_id="tenant-1",
+                user_id="user-1",
+                request=CalendarEventRequest(
+                    summary="x" * 501,
+                    start_at=datetime(2026, 7, 20, 9, tzinfo=timezone.utc),
+                ),
+            )
+
+
+if __name__ == "__main__":
+    unittest.main()