Fix OCR for mixed text and scanned PDF pages

[haosenwang1018]

Issue

Closes #1791

Root cause

When an OCR-enabled PDF has at least one extractable text page, the converter builds non-empty markdown from that page and never reaches the whole-document scanned-PDF fallback. Later pages that have no embedded image objects and no text layer are left as page headers only.

Fix

Render and OCR an individual page when OCR is enabled, no embedded images are detected, and the page text layer is empty. This keeps text-layer pages on the existing path while recovering mixed scanned pages.

Tests

• uv run --project packages/markitdown-ocr --with-editable packages/markitdown --with pytest python -m pytest packages/markitdown-ocr/tests/test_pdf_converter.py -k 'mixed_text_and_scanned_pages or scanned_report or scanned_fallback_format'
• Note: running the full test_pdf_converter.py file in this local uv environment gives 13/14 passing; the remaining existing test_pdf_multipage expectation assumes pdfplumber fails on that fixture, but pdfplumber parses it successfully here before this change.

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