[Fix] Harden PDF and Lua utility functions

[Fix] Harden PDF and Lua utility functions

- lua_util_to_utf8: prevent integer overflow in buffer calculation,
  check for negative length from ucnv_convert
- lua_task_inject_part: validate all table entries before processing
- pdf.lua: refactor recursive flatten to iterative with depth limit,
  add input validation in apply_pdf_filter, reduce timeout to 2s,
  skip text extraction on timeout as results would be incomplete