Fix structured attributes in text and LaTeX publishers

[kocaemre]

Summary

• Handle structured attributes.publish mapping entries in text and LaTeX publishers instead of passing dicts to item.attribute().
• Share simple publish-entry parsing/rendering in BasePublisher for non-HTML tabular output.
• Add regression coverage for text and LaTeX output using the documented fields mapping syntax.

Why

Fixes #841.

Test plan

• RED: uv run pytest doorstop/core/publishers/tests/test_publisher_text.py::TestModule::test_lines_text_item_with_structured_publish_attribute doorstop/core/publishers/tests/test_publisher_latex.py::TestPublisherModule::test_lines_latex_item_with_structured_publish_attribute -q failed before the fix with TypeError: cannot use 'dict' as a dict key.
• GREEN: uv run pytest doorstop/core/publishers/tests/test_publisher_text.py::TestModule::test_lines_text_item_with_structured_publish_attribute doorstop/core/publishers/tests/test_publisher_latex.py::TestPublisherModule::test_lines_latex_item_with_structured_publish_attribute -q → 2 passed.
• uv run pytest doorstop/core/publishers/tests/test_publisher_text.py doorstop/core/publishers/tests/test_publisher_latex.py -q → 32 passed.
• uv run python -m py_compile doorstop/core/publishers/base.py doorstop/core/publishers/text.py doorstop/core/publishers/latex.py doorstop/core/publishers/tests/test_publisher_text.py doorstop/core/publishers/tests/test_publisher_latex.py
• git diff --check

Notes

• uv run pytest doorstop/core/publishers/tests -q was attempted: 128 passed, 1 existing HTML linkify assertion failed outside this change (test_lines_html_item_linkify, mock href escaping difference under this local Python 3.14 environment).
• AI-assisted implementation; I reviewed the diff and ran the checks above.

[kocaemre]

Follow-up: fixed the Black formatting drift that made the CI make check jobs fail.

Verification after the formatting fix:

• poetry run pytest doorstop/core/publishers/tests/test_publisher_text.py doorstop/core/publishers/tests/test_publisher_latex.py -q → 32 passed
• make check → passed (isort, black, mypy, pylint, pydocstyle)
• make test → 883 passed, 1 warning
• git diff --check → passed