feat: add tests and better model validation for gesture commands

ref: N25B-334

src/control_backend/schemas/ri_message.py

@@ -1,7 +1,7 @@
 from enum import Enum
-from typing import Any
+from typing import Any, Literal
 
-from pydantic import BaseModel
+from pydantic import BaseModel, model_validator
 
 
 class RIEndpoint(str, Enum):
@@ -48,5 +48,17 @@ class GestureCommand(RIMessage):
     :ivar data: The id of the gesture to be executed.
     """
 
-    endpoint: RIEndpoint = RIEndpoint(RIEndpoint.GESTURE_TAG) or RIEndpoint(RIEndpoint.GESTURE_TAG)
+    endpoint: Literal[  # pyright: ignore[reportIncompatibleVariableOverride] - We validate this stricter rule ourselves
+        RIEndpoint.GESTURE_SINGLE, RIEndpoint.GESTURE_TAG
+    ]
     data: str
+
+    @model_validator(mode="after")
+    def check_endpoint(self):
+        allowed = {
+            RIEndpoint.GESTURE_SINGLE,
+            RIEndpoint.GESTURE_TAG,
+        }
+        if self.endpoint not in allowed:
+            raise ValueError("endpoint must be GESTURE_SINGLE or GESTURE_TAG")
+        return self