Strict type hints for MQTT integration (#82317)

* Strict type hints for MQTT integration

* Fix errors

* Additional corrections

* Use cv.template to avoid untyped calls

* Enable strict typing policy for MQTT integration

* Use ignore[no-untyped-call]

* Use # type: ignore[unreachable]

* Correct cast

* Refactor getting discovery_payload

* Remove unused type ignore comments

mypy.ini

@@ -1623,6 +1623,16 @@ disallow_untyped_defs = true
 warn_return_any = true
 warn_unreachable = true
 
+[mypy-homeassistant.components.mqtt.*]
+check_untyped_defs = true
+disallow_incomplete_defs = true
+disallow_subclassing_any = true
+disallow_untyped_calls = true
+disallow_untyped_decorators = true
+disallow_untyped_defs = true
+warn_return_any = true
+warn_unreachable = true
+
 [mypy-homeassistant.components.mysensors.*]
 check_untyped_defs = true
 disallow_incomplete_defs = true