Add tests of legacy entity without platform writing state (#148109)

tests/helpers/test_entity.py

@@ -2713,6 +2713,41 @@ async def test_platform_state(
     assert hass.states.get("test.test") is None
 
 
+async def test_platform_state_no_platform(hass: HomeAssistant) -> None:
+    """Test platform state for entities which are not added by an entity platform."""
+
+    class MockEntity(entity.Entity):
+        entity_id = "test.test"
+
+        def async_set_state(self, state: str) -> None:
+            self._attr_state = state
+            self.async_write_ha_state()
+
+    ent = MockEntity()
+    ent.hass = hass
+    assert hass.states.get("test.test") is None
+
+    # The attempt to write when in state NOT_ADDED should be allowed
+    assert ent._platform_state == entity.EntityPlatformState.NOT_ADDED
+    ent.async_set_state("not_added")
+    assert hass.states.get("test.test").state == "not_added"
+
+    # The attempt to write when in state ADDING should be allowed
+    ent._platform_state = entity.EntityPlatformState.ADDING
+    ent.async_set_state("adding")
+    assert hass.states.get("test.test").state == "adding"
+
+    # The attempt to write when in state ADDED should be allowed
+    ent._platform_state = entity.EntityPlatformState.ADDED
+    ent.async_set_state("added")
+    assert hass.states.get("test.test").state == "added"
+
+    # The attempt to write when in state REMOVED should be ignored
+    ent._platform_state = entity.EntityPlatformState.REMOVED
+    ent.async_set_state("removed")
+    assert hass.states.get("test.test").state == "added"
+
+
 async def test_platform_state_fail_to_add(
     hass: HomeAssistant, entity_registry: er.EntityRegistry
 ) -> None: