Add streaming PDF page loading and improve PDF rendering

[ShayDuane]

• Add iterator-based page loading in PageLoader and use it in the async pipeline data-loading thread
• Introduce pdf_to_images_pil_iter and make PDF rendering more robust with proper resource cleanup in image_utils On branch feature/pdf
  Changes to be committed:
  modified: glmocr/dataloader/page_loader.py
  modified: glmocr/pipeline/pipeline.py
  modified: glmocr/utils/image_utils.py

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[Copilot]

Pull request overview

This PR adds streaming (iterator-based) PDF page loading so the async pipeline can begin layout/recognition work before the full PDF is rendered, and it updates PDF rendering utilities to more aggressively clean up pdfium resources.

Changes:

• Add pdf_to_images_pil_iter() generator and update pdf_to_images_pil() to explicitly close pages/documents.
• Add PageLoader.iter_pages_with_unit_indices() streaming API (with PDF streaming support via _iter_pdf).
• Switch the async pipeline data-loading thread to consume the new streaming API.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
glmocr/utils/image_utils.py	Adds generator-based PDF rendering and introduces explicit page/document cleanup.
glmocr/dataloader/page_loader.py	Adds streaming page iteration with unit indices; adds streaming PDF path.
glmocr/pipeline/pipeline.py	Uses the streaming iterator in the async loading thread to enqueue pages as they arrive.

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