test(calendar): TZ-aware datetime in deletable_event_uid fixture

Carry-over from PR #1415 — use datetime.now(UTC) instead of naive
datetime.now() in the fixture so ISO 8601 strings stay unambiguous
when the test runner and HA instance are in different timezones.
Matches the convention in tests/src/e2e/workflows/core/test_history.py.

Pre-existing naive datetime.now() calls elsewhere in this file are
unchanged — format-debt tracked broadly at #1318.

Author: kingpanther13

tests/src/e2e/workflows/calendar/test_calendar.py

@@ -13,7 +13,7 @@
 
 import logging
 import uuid
-from datetime import datetime, timedelta
+from datetime import UTC, datetime, timedelta
 
 import pytest
 
@@ -328,7 +328,10 @@ async def deletable_event_uid(self, mcp_client):
             pytest.skip("No writable calendar found for testing")
 
         summary = f"E2E Deletable Test Event {uuid.uuid4().hex[:8]}"
-        now = datetime.now()
+        # TZ-aware so ISO 8601 strings stay unambiguous when the test runner
+        # and the HA instance are in different timezones (matches the
+        # convention in tests/src/e2e/workflows/core/test_history.py).
+        now = datetime.now(UTC)
         start = (now + timedelta(days=1)).replace(
             hour=14, minute=0, second=0, microsecond=0
         )