fix: switch skip_infer_table_types default to None

[TrueFurina]

Summary

Closes #3063: switch the skip_infer_table_types default from a hard-coded list to None, and set the actual default inside partition().

Changes

• unstructured/partition/auto.py:
  • partition() signature: skip_infer_table_types: Optional[list[str]] = None (was ["pdf", "jpg", "png", "heic"])
  • Inside partition(): normalize None → ["pdf", "jpg", "png", "heic"] before it reaches decide_table_extraction(), so callers can distinguish an explicit value from the default

Verification

• ast.parse passes
• Local assertions cover 3 scenarios:
  • Default (None): pdf skipped for table inference, docx inferred (unchanged behavior)
  • Explicit []: pdf inferred too (no types skipped)
  • Custom ["docx"]: docx skipped, pdf inferred
• Behavior for callers that pass an explicit list is byte-for-byte unchanged