Merges 5 main-side mlx fixes (unslothai#673 zero-token CCE, unslothai#679 + unslothai#692 LoRA save
metadata, unslothai#682 invalid label NaN-poisoning, unslothai#688 tool mask). All 13
conflict regions in unsloth_zoo/mlx/utils.py resolved to keep PR unslothai#684's
behavior where it conflicts on semantics:

  - half-open `<` length mask (PR unslothai#684 fix) wins over main's inclusive `<=`
  - `if labels is None` branch preserved (PR unslothai#684 generality) alongside
    main's `_normalize_cce_label_dtype` dtype widening
  - `_get_image_token_ids` legacy wrapper kept alongside main's new
    `_normalize_cce_label_dtype` / `_normalize_numpy_cce_labels`
  - `_mask_label_token_ids` calls `_normalize_cce_label_dtype` first so
    image masking honors main's uint-widening contract
  - HEAD's `_expand_token_replacements` dropped; main's three-function
    split (`_normalize_numpy_cce_labels` + `_expand_image_token_sequences`
    + `_expand_token_runs`) is canonical; duplicate HEAD wrappers removed
  - `_collate_vlm_prompt_completion_batch` reads back the masked labels
    in int64 so image + attention masking survives without narrowing
  - prompt-completion VLM collator routes through `_apply_vlm_label_masks`
    after dtype normalisation so ignore_token_ids and wide invalid ids
    both reach runtime CCE intact
  - `_to_mx_vlm_batch` uses main's `_normalize_cce_label_dtype` for labels
    while keeping PR unslothai#684's token_type_ids / mm_token_type_ids handling
  - `_unsloth_*` prefix filter preserved so the new collated_position_ids
    flag and main's raw-input-ids carrier both get stripped

152 MLX tests pass post-merge.