Fix AbstractPusTm compatibility regression

Allow `AbstractPusTm` subclasses that implement either
`subservice` (PUS A) or `message_subtype` (PUS C).
This restores PUS A TM instantiation while keeping the
PUS C rename and alias migration path.

---

Generated with the help of Codex (GPT-5)

Co-Authored-By: OpenAI Codex <noreply@openai.com>

Author: mokagio

src/spacepackets/ecss/tm.py

@@ -79,16 +79,10 @@ def timestamp(self) -> bytes:
         pass
 
     @property
-    @abstractmethod
     def message_subtype(self) -> int:
-        pass
+        return self.subservice
 
     @property
-    @deprecation.deprecated(
-        deprecated_in=DEPRECATED_IN_SUBSERVICE_ALIAS,
-        details="use message_subtype property instead",
-        current_version=get_version(),
-    )
     def subservice(self) -> int:
         return self.message_subtype