fix(backend): expose c2_task.source in GET /c2/tasks response

Serialize source as t.source.value (string) in list_simulation_tasks.
Updated test_c2_tasks_list shape assertion to include 'source' and
assert value is 'mimic' for execute-created tasks. Added test in
test_c2_import to assert source='import' in GET /c2/tasks after import.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/test_c2_tasks_list.py

@@ -117,8 +117,9 @@ class TestListTasksHappyPath:
         task = resp.get_json()["tasks"][0]
         for field in ("id", "mythic_task_display_id", "callback_display_id",
                       "command", "params", "status", "completed", "output",
-                      "mapping_applied", "created_at", "completed_at"):
+                      "source", "mapping_applied", "created_at", "completed_at"):
             assert field in task, f"missing field: {field}"
+        assert task["source"] == "mimic"
 
     def test_first_poll_returns_submitted(
         self, client: FlaskClient, admin_token: str