Fix crash when handling a stream after non-stream output

[krassowski]

• Fixes Crash when executing a cell with an output which does not have a name #47
• Add a failing test case (https://github.com/datalayer/jupyter-server-nbmodel/actions/runs/16378348322/job/46283712116#step:7:424):

  Traceback (most recent call last):
    File "/home/runner/work/jupyter-server-nbmodel/jupyter-server-nbmodel/jupyter_server_nbmodel/actions.py", line 276, in kernel_worker
      results[uid] = await _execute_snippet(
                     ^^^^^^^^^^^^^^^^^^^^^^^
    File "/home/runner/work/jupyter-server-nbmodel/jupyter-server-nbmodel/jupyter_server_nbmodel/actions.py", line 213, in _execute_snippet
      reply = await ensure_async(
              ^^^^^^^^^^^^^^^^^^^
    File "/opt/hostedtoolcache/Python/3.12.11/x64/lib/python3.12/site-packages/jupyter_core/utils/__init__.py", line 197, in ensure_async
      result = await obj
               ^^^^^^^^^
    File "/opt/hostedtoolcache/Python/3.12.11/x64/lib/python3.12/site-packages/jupyter_client/client.py", line 565, in _async_execute_interactive
      output_hook(msg)
    File "/home/runner/work/jupyter-server-nbmodel/jupyter-server-nbmodel/jupyter_server_nbmodel/actions.py", line 110, in _output_hook
      if (not cell_outputs) or (cell_outputs[-1]["name"] != output["name"]):
                                ~~~~~~~~~~~~~~~~^^^^^^^^
  KeyError: 'name'

• Add a fix

[github-actions]

👈 Launch a Binder on branch krassowski/jupyter-server-nbmodel/fix-cell-outputs

[echarles]

@krassowski Worth rebasing on #50 to get a green CI

[echarles]

LGTM Thx @krassowski