Fix potential lost logcontext when PerDestinationQueue.shutdown(...) (#19178)

Spawning from looking at the logs in
https://github.com/element-hq/synapse/issues/19165#issuecomment-3527452941
which mention the `federation_transaction_transmission_loop`. I don't
think it's the source of the lost logcontext that person in the issue is
experiencing because this only applies when you try to `shutdown` the
homeserver.

Problem code introduced in
https://github.com/element-hq/synapse/pull/18828

To explain the fix, see the [*Deferred
callbacks*](3b59ac3b69/docs/log_contexts.md (deferred-callbacks))
section of our logcontext docs for more info (specifically using
solution 2).

changelog.d/19178.misc

@@ -0,0 +1 @@
+Fix potential lost logcontext when `PerDestinationQueue.shutdown(...)`.

synapse/federation/sender/per_destination_queue.py

@@ -41,6 +41,7 @@ from synapse.events import EventBase
 from synapse.federation.units import Edu
 from synapse.handlers.presence import format_user_presence_state
 from synapse.logging import issue9533_logger
+from synapse.logging.context import PreserveLoggingContext
 from synapse.logging.opentracing import SynapseTags, set_tag
 from synapse.metrics import SERVER_NAME_LABEL, sent_transactions_counter
 from synapse.types import JsonDict, ReadReceipt
@@ -186,7 +187,8 @@ class PerDestinationQueue:
         self._transaction_manager.shutdown()
         try:
             if self.active_transmission_loop is not None:
-                self.active_transmission_loop.cancel()
+                with PreserveLoggingContext():
+                    self.active_transmission_loop.cancel()
         except Exception:
             pass