perf: lazy page rendering in convert_pdf_to_image

[KRRT7]

Summary

• Render and save each PDF page inside the page loop instead of accumulating all images in memory first, reducing peak memory usage
• When path_only=True, images are no longer retained after saving to disk

Peak memory drops from O(N pages) to O(1 page).

Benchmark

10 PDFs (2,905 total pages), hi_res strategy:

	Before	After
Avg memory	7.5 GB	4.6 GB
Steady-state	~7.9 GB (never released)	~4.8 GB

Test plan

• Added tests for convert_pdf_to_image covering the no-output-folder and output-folder-with-images return paths

[KRRT7]

@badGarnet @qued could you review this when you get a chance?