fix(authz): use grpc.Future isinstance check for async resolution

getattr approach didn't work — UnaryUnaryCall doesn't expose
result via getattr. Use isinstance(call, grpc.Future) instead.

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

Author: ImTotem

src/bcsd_api/authz/client.py

@@ -40,9 +40,10 @@ def _update(operation, res_type: str, res_id: str, relation: str, user_id: str)
 
 
 def _resolve(call):
-    resolve = getattr(call, "result", None)
-    if resolve:
-        return resolve()
+    from grpc import Future
+
+    if isinstance(call, Future):
+        return call.result()
     return call