Fix __len__ of Sliding Sync PerConnectionState ignoring account data (#20124)

Noticed while working on
https://github.com/element-hq/synapse/pull/20003, submitting separately.

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---------

Co-authored-by: Eric Eastwood <erice@element.io>

changelog.d/20124.misc

@@ -0,0 +1 @@
+Fix cache `__len__` of Sliding Sync `PerConnectionState` ignoring account data entries.

synapse/types/handlers/sliding_sync.py

@@ -919,6 +919,7 @@ class PerConnectionState:
         receipts: The status of each room for the receipts stream.
         room_configs: Map from room_id to the `RoomSyncConfig` of all
             rooms that we have previously sent down.
+        account_data: The status of each room for the account_data stream.
     """
 
     last_used_ts: int | None = None
@@ -951,7 +952,12 @@ class PerConnectionState:
         )
 
     def __len__(self) -> int:
-        return len(self.rooms) + len(self.receipts) + len(self.room_configs)
+        return (
+            len(self.account_data)
+            + len(self.rooms)
+            + len(self.receipts)
+            + len(self.room_configs)
+        )
 
 
 @attr.s(auto_attribs=True)