fix(ipynb): label code fences with the notebook's own language

[kevin (kevin9327)]

Converting an R notebook produces

```R
plot(cars)
```

as

```python
plot(cars)
```

The same happens to Julia, Scala, C#, Bash and every other kernel: IpynbConverter._convert writes the fence with a hard-coded python info string, whatever the notebook says it is. The output then tells a reader — and any downstream renderer or model — that the code is Python.

nbformat records the notebook's language in metadata.language_info.name, which is the field nbconvert reads; metadata.kernelspec.language is the kernel's own declaration and is all some producers write. This reads the first, falls back to the second, and falls back to python when a notebook declares neither, which is what the converter has always emitted.

Only the first word of the declaration is used, since a fence's info string ends at the first whitespace.

Reproduction

New test file, on unmodified main (packages/markitdown):

$ python -m pytest tests/test_ipynb_code_language.py -q
FAILED tests/test_ipynb_code_language.py::test_language_info_names_the_fence
FAILED tests/test_ipynb_code_language.py::test_kernelspec_language_is_used_when_language_info_is_absent
2 failed, 2 passed in 2.61s

with the failure showing the wrong label:

>       assert "```R\n" + SOURCE in markdown
E       AssertionError: assert ('```R\n' + 'plot(cars)\n') in '```python\nplot(cars)\n\n```'

The two tests that pass before and after are the ones that pin the existing behaviour: a notebook declaring python, and a notebook declaring no language at all, both still get python.

With the fix:

$ python -m pytest tests/test_ipynb_code_language.py -q
4 passed in 2.22s

Verification

tests/test_module_vectors.py includes the end-to-end test_notebook.ipynb vector, which requires ```python — that notebook declares language_info.name: "python", so it keeps its label.

$ python -m pytest tests/test_module_vectors.py tests/test_module_misc.py -q \
    -k "not file_uris and not case_insensitive and not speech"
183 passed, 2 skipped, 3 deselected in 63.85s   # origin/main
183 passed, 2 skipped, 3 deselected in 61.26s   # this branch

(The three deselected tests fail on main on this Windows machine for reasons unrelated to notebooks: console encoding, file-URI paths, and a missing ffmpeg.)

black reports both touched files unchanged.