refactor: make dpi explicit on convert_pdf_to_image for dedup with unstructured

[KRRT7]

Summary

• Make dpi an explicit parameter (default 200) on convert_pdf_to_image instead of reading inference_config.PDF_RENDER_DPI internally
• Enables unstructured to import and call this function directly, eliminating the duplicate _render_pdf_pages implementation
• No behavior change — both internal callers already pass dpi explicitly

Changelog

## 1.6.2

### Enhancement
- Make `dpi` an explicit parameter on `convert_pdf_to_image` (default 200) instead of reading from config internally, enabling unstructured to use this as the single source of truth for PDF rendering

Depends on / blocks

• Blocks refactor: deduplicate PDF rendering by delegating to unstructured-inference unstructured#4315 (dedup of PDF rendering)
• Blocks Unstructured-IO/core-product#1480 (version bump)

[KRRT7]

Added a second commit that removes PDF_RENDER_DPI from InferenceConfig — it's now dead code since convert_pdf_to_image takes dpi as an explicit parameter instead of reading from config. Callers (i.e. unstructured) pass their own config value directly.