Keep the entry loaded when the last zone is deleted

Deleting the only zone in the UI reloaded the entry with no zones, and
min() over an empty set of models raised during setup, so the entry
failed to load. Counts and learning progress now default to 0, and the
status reads "no_zones" until a zone is added again; adding one resumes
normally. HA Core regression test included.

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

tests/ha/test_lifecycle.py

@@ -290,3 +290,18 @@ async def test_learning_progress_and_last_sample_sensors(hass, entry, inputs, fr
     last = hass.states.get(last_id)
     assert last.state == "waiting_for_inputs"
     assert last.attributes["counts_since_start"] == {"first_sample": 1, "learned": 1, "waiting_for_inputs": 1}
+
+
+async def test_deleting_the_last_zone_keeps_the_entry_loaded(hass, entry, inputs, caplog):
+    await setup(hass, entry)
+    assert hass.config_entries.async_remove_subentry(entry, zone_subentry(entry, "sensor.room").subentry_id)
+    await hass.async_block_till_done()
+    assert entry.state.name == "LOADED"
+    coordinator = hass.data[DOMAIN][entry.entry_id]
+    assert (coordinator.rooms, coordinator.data["status"]) == ([], "no_zones")
+    assert coordinator.data["learning_progress"] == 0
+    assert "Error setting up entry" not in caplog.text
+    # Adding a zone back resumes normally.
+    add_zone(hass, entry, "sensor.room", ["sensor.room"])
+    await hass.async_block_till_done()
+    assert hass.data[DOMAIN][entry.entry_id].data["status"] == "learning"