Align PDF inference image mode

[CooperSheroy]

Summary

• default batch inference to base image mode for PDF inputs
• reject explicit --pdf with unsupported gundam image mode
• update the README PDF example to use base

Validation

• uv run --python 3.12 --with requests python -m py_compile infer.py
• uv run --python 3.12 --with requests python - <<'PY'
  import sys
  import infer

sys.argv = ['infer.py', '--image_dir', 'images']
args = infer.parse_args()
print(f'image_dir_default={args.image_mode}')

sys.argv = ['infer.py', '--pdf', 'doc.pdf']
args = infer.parse_args()
print(f'pdf_default={args.image_mode}')

sys.argv = ['infer.py', '--pdf', 'doc.pdf', '--image_mode', 'gundam']
try:
infer.parse_args()
except SystemExit as exc:
print(f'pdf_gundam_exit={exc.code}')
else:
raise SystemExit('expected parser error')
PY