fix: repair docx zip header casing

[he-yufeng]

Fixes #1812.

Some DOCX producers write ZIP entries whose central-directory name differs from the local file header only by path casing. Python's zipfile rejects those files during extraction even though Word and common zip tools tolerate them.

This patch repairs that narrow case in memory before the existing DOCX pre-processing step reads each member. The central-directory name remains authoritative, and the patch only applies when the local and central names have the same byte length and differ only by case.

Validation:

• .venv\Scripts\python.exe -m pytest packages\markitdown\tests\test_module_misc.py::test_docx_preprocess_repairs_case_mismatched_zip_names -q
• .venv\Scripts\python.exe -m pytest packages\markitdown\tests\test_module_misc.py::test_docx_comments packages\markitdown\tests\test_module_misc.py::test_docx_equations packages\markitdown\tests\test_module_misc.py::test_docx_preprocess_repairs_case_mismatched_zip_names -q
• .venv\Scripts\python.exe -m py_compile packages\markitdown\src\markitdown\converter_utils\docx\pre_process.py packages\markitdown\tests\test_module_misc.py
• .venv\Scripts\black.exe --check packages\markitdown\src\markitdown\converter_utils\docx\pre_process.py packages\markitdown\tests\test_module_misc.py
• git diff --check