convert_hf_to_gguf.py: Refactor how --mtp / --no-mtp is enabled for a model

[tarruda]

Overview

--no-mtp/--mtp doesn't currently work on Hy3 (#25395) despite the model supporting MTP.

Refactor how that those options are added by adding a flag to the ModelBase class and enabling for Qwen 3.x, Stepfun and Hy3.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: I used codex to show me how this mtp/no-mtp option was enabled and refactored the code myself.

[satindergrewal]

Confirming from the Hy3 side: HYV3Model already implements the full no_mtp / mtp_only filtering (the n_nextn hparams guard in set_gguf_parameters plus the filter_tensors checks in conversion/hunyuan.py), so the hardcoded gate in main() was the only thing blocking it. That was my omission in #25395, thanks for catching it. The supports_mtp_export class flag is also a cleaner shape than the growing issubclass chain, and the diff covers all three current MTP families. LGTM.