Wrong retention policy being used when filtering events (lint ControlVarUsedAfterBlockViolation WPS441) (#17272)

Fix loop var being used outside block.

Before this change, we were always using the last room_id's retention policy for all events being filtered.

I found this bug with the [new lint rule, `ControlVarUsedAfterBlockViolation` `WPS441`](https://github.com/astral-sh/ruff/pull/11769), that I re-implemented in `ruff`. Shout-out to @reivilibre for all the help in the beginning! 

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changelog.d/17272.bugfix

@@ -0,0 +1 @@
+Fix wrong retention policy being used when filtering events.

synapse/visibility.py

@@ -151,7 +151,7 @@ async def filter_events_for_client(
             filter_send_to_client=filter_send_to_client,
             sender_ignored=event.sender in ignore_list,
             always_include_ids=always_include_ids,
-            retention_policy=retention_policies[room_id],
+            retention_policy=retention_policies[event.room_id],
             state=state_after_event,
             is_peeking=is_peeking,
             sender_erased=erased_senders.get(event.sender, False),