Tab completion not working in same cell

I create two cells, run both


Then I try tab completion


If I press tab at the end of the second cell, nothing happens. If I press tab at the end of the third cells, it works.

Python repro:
```python
from ipykernel.tests.utils import new_kernel, wait_for_idle, execute, TIMEOUT

cells = [
'import datetime',
't = datetime.date.today()',
't = datetime.date.today()\nt.',
't.'
]

with new_kernel() as kc:
execute(cells[0], kc=kc)
wait_for_idle(kc)
execute(cells[1], kc=kc)
wait_for_idle(kc)
kc.complete(cells[2])
matches_1 = kc.get_shell_msg(block=True, timeout=TIMEOUT)['content']['matches']
kc.complete(cells[3])
matches_2 = kc.get_shell_msg(block=True, timeout=TIMEOUT)['content']['matches']
```

Start by running the Python repro with ipykernel.tests.utils, focusing on kc.complete and the two returned matches values. Trace the kernel completion handling for cells containing a newline and compare it with the single-line case. Done means completion works at the end of the second cell and the regression is covered by a test.

Rédigé par le modèle d'indexation à partir du texte de l'issue.