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/step3.py

@@ -98,6 +98,7 @@ class Step3VLTextModel(Qwen3Model):
 @ModelBase.register("Step3p5ForCausalLM", "Step3p7ForConditionalGeneration")
 class Step35Model(TextModel):
     model_arch = gguf.MODEL_ARCH.STEP35
+    supports_mtp_export = True
 
     # The --mtp / --no-mtp toggles are ModelBase.mtp_only / no_mtp (set in
     # convert_hf_to_gguf.py main()). Unlike Qwen3.5, which stores MTP under a