refactor: Optimize success count calculation in BatchResponse (#837)

Co-authored-by: Lahiru Maramba <llahiru@gmail.com>
Co-authored-by: Jonathan Edey <145066863+jonathanedey@users.noreply.github.com>

Author: 3 people

firebase_admin/messaging.py

@@ -323,7 +323,7 @@ class BatchResponse:
 
     def __init__(self, responses):
         self._responses = responses
-        self._success_count = len([resp for resp in responses if resp.success])
+        self._success_count = sum(1 for resp in responses if resp.success)
 
     @property
     def responses(self):