fix: recover PDF text after inline images

[he-yufeng]

Summary

• add a lazy PyMuPDF text-recovery pass for PDFs with inline image operators
• keep PyMuPDF optional: no default dependency change, and the fallback only runs when the package is installed
• warn instead of silently returning likely partial text when inline-image recovery is needed but PyMuPDF is unavailable
• add regression coverage for both the recovery and warning paths

Addresses #1870.

Test plan

• python -m pytest packages/markitdown/tests/test_pdf_memory.py -q -k "inline_image"
• python -m pytest packages/markitdown/tests/test_pdf_memory.py packages/markitdown/tests/test_pdf_tables.py -q
• python -m py_compile packages/markitdown/src/markitdown/converters/_pdf_converter.py packages/markitdown/tests/test_pdf_memory.py
• python -m mypy --ignore-missing-imports packages/markitdown/src/markitdown/converters/_pdf_converter.py packages/markitdown/tests/test_pdf_memory.py
• python -m pip install -e "packages/markitdown[pymupdf]"
• git diff --check