fix: add empty-ID guard to delete_model to prevent wiping engine models directory

[Copilot]

delete_model did not validate empty engine_id/model_id before path construction. With an empty model_id, _get_model_root resolves to the engine's entire models root directory — which exists — so shutil.rmtree silently deletes every model for that engine. delete_dataset already had this guard; delete_model did not.

Changes

• src/voxkit/storage/models.py — Added early-return guard at the top of delete_model, matching the delete_dataset pattern:

  if not engine_id or not model_id:
      return False, "Engine ID and Model ID cannot be empty."

  Updated docstring Notes to document the validation.

• tests/storage/test_models.py — Added test_delete_model_empty_ids covering: empty model_id, empty engine_id, and both empty.

Testing

• Test testable logic
• Edge cases addressed

Testing Notes: New test covers all three empty-ID permutations (empty model_id only, empty engine_id only, both empty), asserting success is False and "cannot be empty" in msg.

Documentation

• README updated (if applicable)
• Code comments added/updated
• User-facing documentation updated (if applicable)

Documentation Notes: Docstring Notes section updated to reflect the new validation behavior.

Checklist

• Code follows project conventions
• No breaking changes (or documented if intentional)
• All CI checks pass

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