fix(authz): use callable(getattr) for gRPC future detection

UnaryUnaryCall doesn't inherit grpc.Future. Check if result
attribute is callable instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ImTotem

src/bcsd_api/authz/client.py

@@ -40,10 +40,8 @@ def _update(operation, res_type: str, res_id: str, relation: str, user_id: str)
 
 
 def _resolve(call):
-    from grpc import Future
-
-    if isinstance(call, Future):
-        return call.result()
+    if callable(getattr(call, "result", None)):
+        return call.result()  # type: ignore[union-attr]
     return call