Add Qwen3.5 MTP speculative decoding

[aleroot]

Proposed changes

Adds Qwen3.5/Qwen3.6 MTP speculative decoding support for text and VLM models.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)