chore: adjust message/command structure and write unit tests

ref: N25B-205

test/unit/schemas/test_ri_message.py

@@ -0,0 +1,35 @@
+import pytest
+from control_backend.schemas.ri_message import RIMessage, RIEndpoint, SpeechCommand
+from pydantic import ValidationError
+
+def valid_command_1():
+    return SpeechCommand(data="Hallo?")
+
+def invalid_command_1():
+    return RIMessage(endpoint=RIEndpoint.PING, data="Hello again.")
+
+def test_valid_speech_command_1():
+    command = valid_command_1()
+    try:
+        RIMessage.model_validate(command)
+        SpeechCommand.model_validate(command)
+        assert True
+    except ValidationError:
+        assert False
+   
+
+def test_invalid_speech_command_1():
+    command = invalid_command_1()
+    passed_ri_message_validation = False
+    try:
+        # Should succeed, still.
+        RIMessage.model_validate(command)
+        passed_ri_message_validation = True
+
+        # Should fail.
+        SpeechCommand.model_validate(command)
+        assert False
+    except ValidationError:
+        assert passed_ri_message_validation
+    
+