chore: bump mlx-swift-lm to pick up the first VLM unit tests (#147)

Points at acb6020 (SharpAI/mlx-swift-lm#49), which adds VLMGemma4Tests — the
first unit tests for anything under MLXVLM. Measured coverage had that half of
the model layer at 0.3% over 13,304 lines; this is the first shape/config/
forward-pass test against MLXVLM/Models/Gemma4.swift, the largest untested file
in the model layer and where SharpAI/mlx-swift-lm#45's changes landed.

No SwiftLM-side code changes; verified the umbrella builds clean against the
bumped pointer.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: solderzzc