Catch traceback from presumably incorrect participant.import

A presumably incorrect participant import should not throw a traceback.
Unfortunately I only have the traceback and not the import file.

INFO:openslides_backend.shared.handlers.base_handler:Performing action participant.import.
[2026-09-14 12:09:31 +0000] [16] [ERROR] Error handling request POST /system/action/handle_request
Traceback (most recent call last):
  File "/usr/local/lib/python3.12/site-packages/gunicorn/workers/gthread.py", line 479, in handle
    keepalive = self.handle_request(req, conn)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/gunicorn/workers/gthread.py", line 678, in handle_request
    respiter = self.wsgi(environ, resp.start_response)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/openslides_backend/http/application.py", line 164, in __call__
    return self.wsgi_application(environ, start_response)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/openslides_backend/http/application.py", line 154, in wsgi_application
    response = self.dispatch_request(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/openslides_backend/http/application.py", line 103, in dispatch_request
    response_body, access_token = view_instance.dispatch(request)
                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/openslides_backend/http/views/base_view.py", line 121, in dispatch
    return func(request)
           ^^^^^^^^^^^^^
  File "/app/openslides_backend/http/views/action_view.py", line 56, in action_route
    response = handle_action_in_worker_thread(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/openslides_backend/action/action_worker.py", line 94, in handle_action_in_worker_thread
    raise action_worker_thread.exception
  File "/app/openslides_backend/action/action_worker.py", line 265, in run
    self.response = self.handler.handle_request(
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/openslides_backend/action/action_handler.py", line 140, in handle_request
    results = self.execute_write_requests(
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/openslides_backend/action/action_handler.py", line 229, in execute_write_requests
    write_requests, data = get_write_requests(*args)
                           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/openslides_backend/action/action_handler.py", line 279, in parse_actions
    write_request, results = self.perform_action(
                             ^^^^^^^^^^^^^^^^^^^^
  File "/app/openslides_backend/action/action_handler.py", line 328, in perform_action
    write_request, results = action.perform(
                             ^^^^^^^^^^^^^^^
  File "/app/openslides_backend/action/actions/meeting_user/mixin.py", line 52, in perform
    return super().perform(action_data, user_id, internal, is_sub_call)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/openslides_backend/action/action.py", line 157, in perform
    self.prefetch(action_data)
  File "/app/openslides_backend/action/actions/user/participant_import.py", line 24, in prefetch
    super().prefetch(action_data)
  File "/app/openslides_backend/action/mixins/import_mixins.py", line 210, in prefetch
    store_id = cast(list[dict[str, Any]], action_data)[0]["id"]
               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
KeyError: 'id'

Start with openslides_backend/action/actions/user/participant_import.py and the failing lookup in openslides_backend/action/mixins/import_mixins.py:210. Reproduce an import action whose data lacks the id key, then verify that the malformed import is handled without propagating a traceback.