bug/<partition_md> does not properly handled title hireachy

Describe the bug
When processing markdown using partition_md.The title 'section' should be under 'Chapter 1' as its parent, but in this case, it is NaN. Or could you please suggest if there is any other parameter I might have missed that could be causing this problem? Thanks.

To Reproduce
Provide a code snippet that reproduces the issue.

markdown_document = """# Title\n\n \
## Chapter 1\n\n \
Hi this is Jim\n\n Hi this is Joe\n\n \
### Section \n\n \
Hi this is Lance \n\n
## Chapter 2\n\n \
Hi this is Molly"""

from unstructured.partition.md import partition_md
from unstructured.staging.base import convert_to_dataframe

# file = r"C:\Users\Cjy\Desktop\fix\out.md"
raw_md_elements = partition_md(text=markdown_document)
df = convert_to_dataframe(raw_md_elements)
df

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
If applicable, add screenshots to help explain your problem.

Environment Info
Please run python scripts/collect_env.py and paste the output here.
This will help us understand more about the environment in which the bug occurred.

Additional context
version: 0.12.5

Start with the partition_md(text=...) entry point and inspect how the markdown headings in the provided reproduction are converted into elements and then into a dataframe by convert_to_dataframe. Confirm the observed NaN parent for “Section” and define the expected heading hierarchy before changing behavior; add or update a focused regression test for the sample markdown.