fix: update Docling call to generate figure caption

[christopherkao]

Description

• PDF ingestion using Docling has been failing to generate captions for figures because the arguments into generate_single_figure_caption have been passed in the wrong order. As a result, the figure was being passed in as the vlm_endpoint, and the vlm_endpoint was being passed in as the figure.
• Fixes [BUG] docling not working: Error generating caption: No connection adapters were found for 'data:image/png;base64 #683

Type of change

• New features (non-breaking change).
• Bug fix (non-breaking change).
• Breaking change (fix or feature that would cause existing functionality not to work as expected).

Checklist

• I have performed a self-review of my code.
• I have added thorough tests if it is a core feature.
• There is a reference to the original bug report and related work.
• I have commented on my code, particularly in hard-to-understand areas.
• The feature is well documented.