fix(mcp): list_maintenance_plans reports only a slot that fired as last_slot_at (#5863)

A new plan's last_slot_at holds the moment it was made, so its first
slot is the next one; the tool returned that moment, a Wednesday
evening for a Sunday plan, beside latest_run: null. cadence.fired_slot
returns it only when it falls on one of the plan's slots.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/plugins/host_agent/test_cadence.py

@@ -21,6 +21,16 @@ def _plan(**kw):
     return NS(**base)
 
 
+def test_a_plan_that_has_not_fired_has_no_last_slot():
+    """#5863: a new plan's last_slot_at is the moment it was made, a Wednesday
+    evening for a Sunday plan; only a slot that fired is reported."""
+    made = _plan(time_zone="America/New_York", last_slot_at=datetime(2026, 10, 14, 23, 54, 58, tzinfo=timezone.utc))
+    assert c.fired_slot(made) is None
+    fired = datetime(2026, 10, 18, 7, 0, tzinfo=timezone.utc)      # Sunday 03:00 New York
+    assert c.fired_slot(_plan(time_zone="America/New_York", last_slot_at=fired)) == fired
+    assert c.fired_slot(_plan()) is None
+
+
 def test_the_last_slot_is_the_latest_weekday_and_time_in_the_plans_zone():
     assert c.last_slot(_plan(), NOW) == datetime(2026, 10, 11, 3, 0, tzinfo=timezone.utc)
     # 03:00 in New York is 07:00 UTC in October (EDT)