fix: always include parameters field in Genesys AudioHook messages

The AudioHook protocol requires every message to carry a `parameters`
object. `_create_message` conditionally included it only when parameters
were truthy, so pong responses and closed responses without
outputVariables were sent without the field.

Clients that validate message structure (including the Genesys reference
implementation) rejected these messages, which broke server sequence
tracking and prevented outputVariables from reaching the Architect flow.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/genesys/test_genesys_serializer.py

@@ -76,6 +76,7 @@ class TestGenesysAudioHookSerializer:
 
         assert msg["type"] == "pong"
         assert msg["id"] == serializer.session_id
+        assert msg["parameters"] == {}
 
     def test_create_closed_response(self):
         """Test creating a closed response message."""
@@ -86,7 +87,7 @@ class TestGenesysAudioHookSerializer:
 
         assert msg["type"] == "closed"
         assert serializer.is_open is False
-        assert "parameters" not in msg  # No parameters when no output_variables
+        assert msg["parameters"] == {}  # Empty parameters when no output_variables
 
     def test_create_closed_response_with_output_variables(self):
         """Test creating a closed response with custom output variables."""