fix: share one 80-char cap for image and table asset filenames

[EricNGOntos]

Summary

• After fix: truncate markdown image name context to prevent OS filename limit #236, image context truncation used a hard-coded 60 while table header naming used a separate _MAX_TABLE_NAME_CHARS = 80.
• Introduce shared MAX_ASSET_FILE_NAME_CHARS = 80 in file_utils and use it for markdown image naming, deferred image rename, and table header sanitization.

Test plan

• Confirm markdown image rename with a long last_context still succeeds (no File name too long)
• Confirm table HTML filenames still truncate at 80 chars via sanitize_table_name_from_header
• Smoke a PDF/markdown parse that produces both images and tables

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