fix(ocr): render scanned PDF pages before embedded images

[Elioooon (Elioooon)]

Summary

Fixes #2343.

• render zero-text PDF pages at 300 DPI and OCR the full page before considering embedded images
• reuse the already-open pdfplumber page for image extraction instead of reopening the whole PDF once per page
• share the existing full-page render/OCR path between per-page and document fallbacks

This keeps mixed PDFs intact: text pages still interleave extracted text and embedded-image OCR, while scanned pages no longer lose their body content to small decorative images.

Verification

• python -m pytest packages/markitdown-ocr/tests/test_pdf_converter.py -q -k 'scanned_page_uses_full_page_ocr or convert_reuses_open_page_for_image_extraction' - 2 passed
• python -m pytest packages/markitdown-ocr/tests -q - 37 passed, 1 pre-existing failure: test_pdf_multipage assumes pdfplumber rejects the fixture, but the current dependency version extracts it successfully; reproduced unchanged on main
• black==23.7.0 --check packages/markitdown-ocr/src/markitdown_ocr/_pdf_converter_with_ocr.py packages/markitdown-ocr/tests/test_pdf_converter.py - passed
• git diff --check - passed

Risk

Scoped to the OCR plugin's PDF path. Text-only conversion and non-PDF converters are unchanged. A page is treated as scanned only when page.extract_text() is empty or whitespace.

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