Add conversation mobile app webhook (#86239)

* Add conversation mobile app webhook

* Re-instate removed unused import which was used as fixture

tests/components/conversation/conftest.py

@@ -0,0 +1,15 @@
+"""Conversation test helpers."""
+
+import pytest
+
+from homeassistant.components import conversation
+
+from . import MockAgent
+
+
+@pytest.fixture
+def mock_agent(hass):
+    """Mock agent."""
+    agent = MockAgent()
+    conversation.async_set_agent(hass, agent)
+    return agent