Fix 'Inbound federation correctly soft fails events'

[erikjohnston]

Example failure:

Error: FAILURE: #714: Inbound federation correctly soft fails events
Logs
  # Started: 2023-11-16 11:06:1[9](https://github.com/matrix-org/synapse/actions/runs/6889752308/job/18741444869?pr=16643#step:8:10).683
  # Ended: 2023-11-16 11:06:20.1[10](https://github.com/matrix-org/synapse/actions/runs/6889752308/job/18741444869?pr=16643#step:8:11)
  # SyTest::Federation::Datastore=HASH(0x560d97f08098) has no event id '$aupmJZ2kG7Td8jeW06Dk-8BcVKzNoNLb56b3OOLlRc0' at tests/50federation/52soft-fail.pl line 120.
  # 0.034925: Registered new user @anon-2023[11](https://github.com/matrix-org/synapse/actions/runs/6889752308/job/18741444869?pr=16643#step:8:12)16_110041-868:localhost:8800
  # 0.290339: Joined room
  # 0.317274: Got join down sync
  # 0.400517: Body
  # { event_id => "\$aupmJZ2kG7Td8jeW06Dk-8BcVKzNoNLb56b3OOLlRc0" }
  # 0.415683: Blocked new messages
  # 0.416053: Sending blocked event $YrtlC2paoPAwqiTPNMfibPyZi9e3dpNn1bhb_RC58gQ
  # {
  #   auth_events      => [
  #                         "\$jFjRiizTsoyTbODG3UOkmzz2a0EKmVLLw0jow9l8DhY",
  #                         "\$o835Uvo15VUvLjOr1M6oKjtIPdXttdFk08e1H6qevvk",
  #                         "\$iCZmJ_8GsxwCrrV4oHsMcbU5FYDcelfeqVuKrrdmIlw",
  #                         "\$j3Jp2vtZcINaMX6Ku-skKGNqxQ0qSBTzEZcfj_9Y5Bc",
  #                       ],
  #   content          => { body => "Denied" },
  #   depth            => JSON::number(8),
  #   hashes           => { sha256 => "3csuXjYw1yMpZs/yjUgWQIPNNVR6FDMXn4895yzfB5k" },
  #   origin_server_ts => JSON::number(1700[13](https://github.com/matrix-org/synapse/actions/runs/6889752308/job/18741444869?pr=16643#step:8:14)2780099),
  #   prev_events      => ["\$j3Jp2vtZcINaMX6Ku-skKGNqxQ0qSBTzEZcfj_9Y5Bc"],
  #   room_id          => "!bfUpdvWJvZvKdsfNvl:localhost:8800",
  #   sender           => "\@__ANON__-96:localhost:39621",
  #   signatures       => {
  #                         "localhost:39621" => {
  #                           "ed255[19](https://github.com/matrix-org/synapse/actions/runs/6889752308/job/18741444869?pr=16643#step:8:20):1" => "di7EqrbADJXChIWd+DofuCQG5pmt77ZN5Lzz9T/hFFG/htLa2ltj2Lm/YEGr80yCbhmv4gQ3rpP2A1OtZkmeBw",
  #                         },
  #                       },
  #   type             => "m.room.message",
  # }