fix(asciidoc): stop a dedented list from crashing the backend

[chuenchen309]

An AsciiDoc list that starts indented and then dedents back to the base level crashes the backend:

AsciiDocBackend(...).convert()   # source: "  * a\n* b\n"
# TypeError: '<' not supported between instances of 'int' and 'NoneType'

The dedent loop in _parse_list_item handling keeps decrementing level and comparing item["indent"] < indents[level], but indents[0] (the base level) is never assigned an int — only indents[level + 1] is set when a list starts or nests deeper. So once the loop walks past level 0 the comparison hits None and raises.

The fix stops the loop at level 0, where there is no enclosing list to dedent out of. Nested and flat lists are unaffected; only the previously-crashing dedent-to-base case changes (it now keeps both items).

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

test_list_dedent_to_base_does_not_crash converts " * a\n* b\n" and asserts both items survive; it fails before this change (TypeError) and passes after. The existing tests/test_backend_asciidoc.py stays green (7 passed); ruff check / ruff format pass.

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Disclosure: this contribution is fully AI-authored and autonomous (Claude Code, acting on this account). An AI found the bug, wrote and ran the repro and the test, and wrote this description; the human account holder reviews every change and is accountable for it. The verification above is real and re-runnable from the diff. If this isn't the kind of contribution you want, say so and I'll close it.

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[dolfim-ibm]

@chuenchen309 thanks for the proposed fix. I think this is needed.

On the other hand, checking the PR against the spec, it seems that we should also fix the parsing according to the AsciiDoc spec.

In the specific

* a
    * b

Per the spec this is a flat two-item list (both single *), but an indentation-driven parser is liable to treat * b as nested under * a, which would be wrong. So the fix is a correct and safe crash patch, but it doesn't address the underlying spec mismatch — it just stops the one dedent-to-base case from throwing.

Do you want to address this here directly? (otherwise we will have some follow up issue for it)