use same logic Tagno as for aioca and p4p callbacks

src/ophyd_async/tango/core/_tango_transport.py

@@ -732,25 +732,21 @@ def set_callback(self, callback: Callback | None) -> None:
                 " for which polling is disabled."
             )
 
+        if callback and self.proxies[self.read_trl].has_subscription():  # type: ignore
+            msg = "Cannot set a callback when one is already set"
+            raise RuntimeError(msg)
+
+        if self.proxies[self.read_trl].has_subscription():  # type: ignore
+            self.proxies[self.read_trl].unsubscribe_callback()  # type: ignore
+
         if callback:
             try:
-                # TODO: remove assert, I fail to get the test to work
-                # as soon as move the assert into a separate RunTime error
-                # the `test_tango_sim` fails
-                assert not self.proxies[self.read_trl].has_subscription()  # type: ignore  # noqa: S101
                 self.proxies[self.read_trl].subscribe_callback(callback)  # type: ignore
-            except AssertionError as ae:
-                raise RuntimeError(
-                    "Cannot set a callback when one is already set"
-                ) from ae
             except RuntimeError as exc:
                 raise RuntimeError(
                     f"Cannot set callback for {self.read_trl}. {exc}"
                 ) from exc
 
-        else:
-            self.proxies[self.read_trl].unsubscribe_callback()  # type: ignore
-
     def set_polling(
         self,
         allow_polling: bool = True,