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/base.py

@@ -109,7 +109,9 @@ class ModelBase:
     sentence_transformers_dense_modules: bool = False
 
     # MTP (multi-token prediction) export modes; set by main() before instantiation.
-    # Architectures opt in by overriding the handling (see _Qwen35MtpMixin).
+    # Architectures that implement the filtering/export behavior opt in by
+    # setting supports_mtp_export = True on their model class or a mixin.
+    supports_mtp_export: bool = False
     mtp_only: bool = False
     no_mtp: bool = False