fix: remove duplicate characters caused by fake bold rendering in PDFs

[bittoby]

Closes #3864

Summary

• Fixes issue where bold text in PDFs is extracted with duplicate characters (e.g., "BOLD" → "BBOOLLDD")
• Some PDF generators simulate bold by rendering each character twice at slightly offset positions
• Added character-level deduplication based on position proximity to detect and remove these duplicates

Problem

When extracting text from certain PDFs, bold text appears duplicated:

# Before fix
elements = partition_pdf("document.pdf", strategy="fast")
print(elements[0].text)  # Output: ">60>60" instead of ">60"

Solution

Added character-level deduplication that:

• Compares consecutive characters' text content and position
• Removes duplicates where same character appears within 3 pixels (configurable)
• Preserves spaces and other non-character elements (LTAnno objects)

# After fix
elements = partition_pdf("document.pdf", strategy="fast")
print(elements[0].text)  # Output: ">60" ✓

Configuration

# Default: 3.0 pixels (enabled)
export PDF_CHAR_DUPLICATE_THRESHOLD=3.0

# Disable deduplication
export PDF_CHAR_DUPLICATE_THRESHOLD=0

# More aggressive deduplication
export PDF_CHAR_DUPLICATE_THRESHOLD=5.0

[bittoby]

@badGarnet Could you please review this PR? Thanks!