add sandbox observability contracts

repository/sandbox_session.py

@@ -29,6 +29,10 @@ class InMemorySandboxSessionRepository(SandboxSessionRepository):
                 if session.expires_at <= now
             ]
 
+    def count_active(self) -> int:
+        with self._lock:
+            return len(self._sessions_by_chat_id)
+
     def save(self, session: SandboxSession) -> None:
         with self._lock:
             self._sessions_by_chat_id[session.chat_id] = session