convert_hf_to_gguf: support split MTP export for HY V3 (#25641)

- Add a supports_mtp_export capability to ModelBase so architectures can opt
  into --mtp and --no-mtp without extending a central class allowlist.
- Enable the capability for the existing Qwen3.5/3.6 and Step3.5/3.7
  implementations, and for HY V3, whose converter already supports
  filtering the appended MTP layers.

conversion/qwen.py

@@ -541,6 +541,7 @@ class _Qwen35MtpMixin:
     `mtp.*` to the standard layer-indexed nextn naming so the existing
     tensor_map handles them."""
 
+    supports_mtp_export = True
     hparams: dict[str, Any]
     model_arch: gguf.MODEL_ARCH
     gguf_writer: gguf.GGUFWriter