Markdown backend keeps a whitespace-only first line in code blocks #4541

Bug

Follow-up to #4485 / #4487. The Markdown backend trims code block content with lstrip("\n").rstrip(), so a first line that holds only spaces survives: the code item starts with a line of whitespace. #4502 fixed the same case in the LaTeX backend (review suggestion from @DanielNg0729) by dropping leading lines that hold only whitespace.

Steps to reproduce

from docling.datamodel.base_models import InputFormat
from docling.document_converter import DocumentConverter

md = "```\n   \n    x = 1\n```\n"
doc = DocumentConverter().convert_string(md, format=InputFormat.MD).document
print([t.text for t in doc.texts if t.label == "code"])

Actual: [' \n x = 1']
Expected: [' x = 1']

Docling version

main @ 51fe9ff

Python version

3.12.14

I'd like to send a PR using the same trimming as #4502.