Fix face landmark batch output existence check

[fallintoplace]

Summary

• replace the missing check_output_exists() call in FaceLandmarkExtractor.process_batch() with the same list_files()-based output guard used by the other batch preprocessors
• raise a clear overwrite error that includes the configured output prefix and object count when force_write=False
• add a regression test that exercises the batch guard without requiring MediaPipe model setup

Root cause

FaceLandmarkExtractor.process_batch() called S3Client.check_output_exists(), but S3Client does not define that method. When force_write=False, batch runs failed immediately with AttributeError before manifest loading.

Validation

• PYTHONPATH=src .venv/bin/pytest tests/preprocessing/face_landmark_extractor/test_extractor.py