Remove ebus_sdk.mqtt shim; import MqttClient directly

The src/ebus_sdk/mqtt.py shim has been re-exporting MqttClient and
AUTH_TYPE_USER_PASS from ebus-mqtt-client since 56ad978 (2026-03-14).
With the upstream package stable on PyPI, drop the shim and point the
in-tree imports at ebus_mqtt_client directly.

Public API is unchanged: `from ebus_sdk import MqttClient` still works.
Only direct `from ebus_sdk.mqtt import ...` would break, and nothing in
this repo does that.

tests/test_mqtt.py is removed as well; it duplicated tests that already
live in ebus-mqtt-client/tests/test_client.py. The two _on_message
dispatch tests that were unique to this file were ported upstream in
ebus-mqtt-client 9f73f82.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dcj

src/ebus_sdk/__init__.py

@@ -40,7 +40,7 @@
 )
 
 # MQTT client
-from .mqtt import MqttClient
+from ebus_mqtt_client import MqttClient
 
 __version__ = "0.1.2"
 

src/ebus_sdk/homie.py

@@ -55,7 +55,7 @@ class StrEnum(str, Enum):
 
 # from deprecated import deprecated
 from typing import Any, Callable, List, Optional, Type, Union
-from .mqtt import MqttClient
+from ebus_mqtt_client import MqttClient
 
 logger = logging.getLogger("homie")
 
@@ -1369,7 +1369,7 @@ def ebus_cfg_add_auth(cfg, username, password):
     """
     Add authentication to the config dictionary
     """
-    from .mqtt import AUTH_TYPE_USER_PASS
+    from ebus_mqtt_client import AUTH_TYPE_USER_PASS
 
     cfg["authentication"] = {
         "type": AUTH_TYPE_USER_PASS,