Fix: Bedrock aspect ratio rejection for form PDFs

[Nishit24113]

Summary

• Form PDFs (court forms, license forms, etc.) contain thin decorative elements (underlines, borders, separators) that Adobe AutoTag extracts as images
• These images have extreme aspect ratios (e.g., 500px wide x 2px tall = 250:1)
• Bedrock API rejects any image with aspect ratio > 20:1
• When ALL images fail, the job crashes via process.exit(1) and Step Function retries indefinitely — user sees it stuck for 60+ minutes

Changes (1 file, no new dependencies)

• Added getImageDimensions() — reads PNG/JPEG headers to get width/height
• Added isAspectRatioValid() — checks against Bedrock's 20:1 limit
• Before sending each image to Bedrock, checks aspect ratio. If > 20:1, skips it and assigns "Decorative element" as alt text
• Updated failure condition: only process.exit(1) if all images failed AND none were skipped

Test plan

• Upload a form PDF with thin horizontal lines (e.g., Form_Example_2.pdf)
• Verify job completes instead of failing
• Verify skipped images get "Decorative element" alt text
• Verify normal images still get AI-generated alt text
• Check CloudWatch logs for skip messages with dimensions