Fix Traceback user.save_saml_account

backendaction: INFO:openslides_backend.shared.handlers.base_handler:Performing action user.save_saml_account.
backendaction: [2026-09-18 09:45:53 +0000] [17] [ERROR] Error handling request POST /internal/handle_request
backendaction: Traceback (most recent call last):
backendaction:   File "/usr/local/lib/python3.12/site-packages/gunicorn/workers/gthread.py", line 516, in handle
backendaction:     return self._keepalive_after(conn, self.handle_request(req, conn))
backendaction:                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
backendaction:   File "/usr/local/lib/python3.12/site-packages/gunicorn/workers/gthread.py", line 712, in handle_request
backendaction:     respiter = self.wsgi(environ, resp.start_response)
backendaction:                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
backendaction:   File "/app/openslides_backend/http/application.py", line 164, in __call__
backendaction:     return self.wsgi_application(environ, start_response)
backendaction:            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
backendaction:   File "/app/openslides_backend/http/application.py", line 154, in wsgi_application
backendaction:     response = self.dispatch_request(request)
backendaction:                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
backendaction:   File "/app/openslides_backend/http/application.py", line 103, in dispatch_request
backendaction:     response_body, access_token = view_instance.dispatch(request)
backendaction:                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
backendaction:   File "/app/openslides_backend/http/views/base_view.py", line 121, in dispatch
backendaction:     return func(request)
backendaction:            ^^^^^^^^^^^^^
backendaction:   File "/app/openslides_backend/http/views/action_view.py", line 72, in internal_action_route
backendaction:     response = handle_action_in_worker_thread(
backendaction:                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
backendaction:   File "/app/openslides_backend/action/action_worker.py", line 94, in handle_action_in_worker_thread
backendaction:     raise action_worker_thread.exception
backendaction:   File "/app/openslides_backend/action/action_worker.py", line 265, in run
backendaction:     self.response = self.handler.handle_request(
backendaction:                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
backendaction:   File "/app/openslides_backend/action/action_handler.py", line 140, in handle_request
backendaction:     results = self.execute_write_requests(
backendaction:               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
backendaction:   File "/app/openslides_backend/action/action_handler.py", line 229, in execute_write_requests
backendaction:     write_requests, data = get_write_requests(*args)
backendaction:                            ^^^^^^^^^^^^^^^^^^^^^^^^^
backendaction:   File "/app/openslides_backend/action/action_handler.py", line 279, in parse_actions
backendaction:     write_request, results = self.perform_action(
backendaction:                              ^^^^^^^^^^^^^^^^^^^^
backendaction:   File "/app/openslides_backend/action/action_handler.py", line 328, in perform_action
backendaction:     write_request, results = action.perform(
backendaction:                              ^^^^^^^^^^^^^^^
backendaction:   File "/app/openslides_backend/action/action.py", line 161, in perform
backendaction:     cast(list[dict[str, Any]], action_data)[i] = self.validate_fields(instance)
backendaction:                                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
backendaction:   File "/app/openslides_backend/action/actions/user/save_saml_account.py", line 150, in validate_fields
backendaction:     self.apply_meeting_mapping(instance, instance_old)
backendaction:   File "/app/openslides_backend/action/actions/user/save_saml_account.py", line 268, in apply_meeting_mapping
backendaction:     for key, value in self.get_field_data(
backendaction:                       ^^^^^^^^^^^^^^^^^^^^
backendaction:   File "/app/openslides_backend/action/actions/user/save_saml_account.py", line 469, in get_field_data
backendaction:     cast(set, result).update(value.split(", "))
backendaction:                              ^^^^^^^^^^^
backendaction: AttributeError: 'list' object has no attribute 'split'

Start in openslides_backend/action/actions/user/save_saml_account.py, especially get_field_data and its call from apply_meeting_mapping. Reproduce the user.save_saml_account action with the failing SAML mapping and trace how the value reaches the split call. Done means the action no longer raises this AttributeError for the reported input.