bug: IpynbConverter loses document title when cell source is a string instead of list

Bug

The nbformat spec allows cell source to be either a list of strings or a plain string. When source is a plain string, IpynbConverter silently produces result.title = None even when the cell starts with a # Heading.

Reproduction

`python
import io, json
from markitdown import MarkItDown

md = MarkItDown()

source as LIST — works correctly

nb_list = {'nbformat': 4, 'nbformat_minor': 5,
'metadata': {'kernelspec': {'name': 'python3', 'display_name': 'Python 3', 'language': 'python'}},
'cells': [{'cell_type': 'markdown', 'source': ['# My Report\n', '\n', 'Content'], 'metadata': {}}]}

source as STRING — same content, valid per nbformat spec

nb_str = {'nbformat': 4, 'nbformat_minor': 5,
'metadata': {'kernelspec': {'name': 'python3', 'display_name': 'Python 3', 'language': 'python'}},
'cells': [{'cell_type': 'markdown', 'source': '# My Report\n\nContent', 'metadata': {}}]}

r1 = md.convert(io.BytesIO(json.dumps(nb_list).encode()), url='a.ipynb')
r2 = md.convert(io.BytesIO(json.dumps(nb_str).encode()), url='b.ipynb')

print(r1.title) # 'My Report' ✓
print(r2.title) # None ✗
`

Root cause

_ipynb_converter.py line 72 does for line in source_lines where source_lines is the raw source value from the cell. When source is a string, this iterates character by character, so line.startswith('# ') never matches.

Fix

Normalise string source to a list before processing:
python source = cell.get('source', []) if isinstance(source, str): source = source.splitlines(keepends=True) source_lines = source

PR: #2113

Start in _ipynb_converter.py around line 72 and reproduce the issue with the string-valued cell source shown in the report. Compare it with the list-valued case and verify that conversion extracts the heading as the document title for both valid source forms.