✅(task) add test for task API and confirm unkown task behaviour

According to Celery's author here:
https://github.com/celery/celery/issues/3596#issuecomment-262102185

There is no way to check if a task ID exists. It will just return
"PENDING" indefinitely. Add tests for this accordingly.

Closes #91

src/backend/core/api/viewsets/task.py

@@ -2,14 +2,13 @@
 
 import logging
 
-from celery.result import AsyncResult
 from celery import states as celery_states
+from celery.result import AsyncResult
 from drf_spectacular.utils import (
     OpenApiExample,
     extend_schema,
     inline_serializer,
 )
-from rest_framework import exceptions as drf_exceptions
 from rest_framework import permissions
 from rest_framework import serializers as drf_serializers
 from rest_framework.response import Response
@@ -66,10 +65,11 @@ class TaskDetailView(APIView):
     def get(self, request, task_id):
         """Get the status of a Celery task."""
 
-        # Check if the task exists
         task_result = AsyncResult(task_id, app=celery_app)
-        if not task_result.id:
-            raise drf_exceptions.NotFound("Task not found")
+
+        # By default unknown tasks will be in PENDING. There is no reliable
+        # way to check if a task exists or not with Celery.
+        # https://github.com/celery/celery/issues/3596#issuecomment-262102185
 
         # Prepare the response data
         result_data = {

src/backend/core/tests/api/test_messages_create.py

@@ -144,9 +144,26 @@ class TestApiDraftAndSendMessage:
 
         assert send_response.status_code == status.HTTP_200_OK
 
-        mock_send_outbound_message.assert_called()
+        # Checks on the returned task_id
+        task_id = send_response.data["task_id"]
+        assert task_id is not None
 
-        # TODO: checks on returned task_id
+        # Check with an unknown task_id
+        task_response = client.get(
+            reverse("task-detail", kwargs={"task_id": "unknown-task-id"})
+        )
+        assert task_response.status_code == status.HTTP_200_OK
+        assert task_response.data["status"] == "PENDING"
+
+        # Call the task API
+        task_response = client.get(reverse("task-detail", kwargs={"task_id": task_id}))
+        assert task_response.status_code == status.HTTP_200_OK
+        assert task_response.data["status"] == "SUCCESS"
+        assert task_response.data["result"] is not None
+        assert task_response.data["result"]["message_id"] == draft_message_id
+        assert task_response.data["error"] is None
+
+        mock_send_outbound_message.assert_called()
 
         sent_message = models.Message.objects.get(id=draft_message_id)
         assert sent_message.raw_mime