Validate steps in Flowhandler (#102152)

* Validate steps in Flowhandler

* Move validation to FlowManager._async_handle_step

* Fix _raise_if_not_has_step

* Fix config_entries tests

* Fix tests

* Rename

* Add test

tests/components/config/test_config_entries.py

@@ -798,6 +798,9 @@ async def test_options_flow(hass: HomeAssistant, client) -> None:
                         description_placeholders={"enabled": "Set to true to be true"},
                     )
 
+                async def async_step_user(self, user_input=None):
+                    raise NotImplementedError
+
             return OptionsFlowHandler()
 
     mock_integration(hass, MockModule("test"))
@@ -1271,6 +1274,9 @@ async def test_ignore_flow(
             await self.async_set_unique_id("mock-unique-id")
             return self.async_show_form(step_id="account")
 
+        async def async_step_account(self, user_input=None):
+            raise NotImplementedError
+
     ws_client = await hass_ws_client(hass)
 
     with patch.dict(HANDLERS, {"test": TestFlow}):