Preserve task markers in indented list code

[AryanRG]

With task lists enabled, checkbox-like text at the start of an indented code block inside a list item is consumed as a task marker. For example, a list item containing literal [x] code loses those characters in both the code token and rendered HTML, and gains a checkbox. GFM task markers apply when the first block is a paragraph, not an indented code block.

Check whether the item is an indented code block after its indentation state is established and before consuming a task marker. This preserves literal code while leaving ordinary task items unchanged. Add boundary regressions and an Unreleased changelog note.

Validation:

• The proposed regressions: 12 failed on the pristine base; 19 focused cases passed with the fix.
• Full local suite: 1,051 passed on Python 3.13.
• Pinned pre-commit hooks, including Ruff and strict mypy, passed.
• Independent differential checks covered 161,280 parser/render comparisons. Other Python versions and hosted CI are not yet verified.

AI assistance was used to investigate, implement, test, and review this change.