Use sender instead of user_id for any stripped state event usages

[MadLittleMods]

Use sender instead of user_id for any stripped state event usages

Spawning from element-hq/synapse#19723 (where these tests started to fail) which ensures we actually use stripped state everywhere. Stripped state only has content, sender, state_key, type available. Previously, this relied on the full PDU being available which meant access touser_id as well.

user_id is a deprecated field from Synapse that isn't in the spec.

Dev notes

Example failure:

Error: FAILURE: #219: Can invite unbound 3pid over federation with no ops into a private room
Logs
  # Started: 2026-05-26 20:15:54.169
  # Ended: 2026-05-26 20:15:55.904
  # No inviter member invite state at tests/30rooms/12thirdpartyinvite.pl line 229.
  # 0.029646: Registered new user @anon-20260526_201439-154:127.0.0.1:8838
  # 0.052488: Registered new user @anon-20260526_201439-152:127.0.0.1:8800
  # 0.053313: Registered new user @anon-20260526_201439-153:127.0.0.1:8800
  # 0.439184: room_id=!KRaggKHbkwunOCEjIy:127.0.0.1:8800
  # 0.545050: Invited user @anon-20260526_201439-153:127.0.0.1:8800 to !KRaggKHbkwunOCEjIy:127.0.0.1:8800
  # {}
  # 0.664036: User @anon-20260526_201439-153:127.0.0.1:8800 joined room
  # { room_id => "!KRaggKHbkwunOCEjIy:127.0.0.1:8800" }
  # 0.714240: Created room_id=!KRaggKHbkwunOCEjIy:127.0.0.1:8800
  # 1.159396: sent 3pid invite for lemurs@monkeyworld.org to SyTest::Identity::Server=HASH(0x55b492174fc8)
  # 1.730745: Invite
  # {
  #   invite_state => {
  #     events => [
  #       {
  #         content   => {
  #                        creator => "\@anon-20260526_201439-152:127.0.0.1:8800",
  #                        room_version => 10,
  #                      },
  #         sender    => "\@anon-20260526_201439-152:127.0.0.1:8800",
  #         state_key => "",
  #         type      => "m.room.create",
  #       },
  #       {
  #         content   => { join_rule => "invite" },
  #         sender    => "\@anon-20260526_201439-152:127.0.0.1:8800",
  #         state_key => "",
  #         type      => "m.room.join_rules",
  #       },
  #       {
  #         content   => { alias => "#test-20260526_201439-23:127.0.0.1:8800" },
  #         sender    => "\@anon-20260526_201439-152:127.0.0.1:8800",
  #         state_key => "",
  #         type      => "m.room.canonical_alias",
  #       },
  #       {
  #         content   => {
  #                        membership => "invite",
  #                        third_party_invite => {
  #                          display_name => "Bob",
  #                          signed => {
  #                            mxid => "\@anon-20260526_201439-154:127.0.0.1:8838",
  #                            signatures => {
  #                              "localhost:45425" => {
  #                                "ed25519:0" => "nYMBeK9UPutCo2CxT7JQat8pwbmmixGJT/xVTqsRZVArP7YVwWv0XRogUYA82pSqoZyw9eLtQ6dvmT4ORuAqAg",
  #                              },
  #                            },
  #                            token => 3,
  #                          },
  #                        },
  #                      },
  #         sender    => "\@anon-20260526_201439-153:127.0.0.1:8800",
  #         state_key => "\@anon-20260526_201439-154:127.0.0.1:8838",
  #         type      => "m.room.member",
  #       },
  #     ],
  #   },
  # }

Testing:

docker run --rm -it \
    -v ~/Documents/github/element/synapse:/src:ro \
    -v ~/Documents/github/element/sytest/logs\:/logs \
    -v ~/Documents/github/element/sytest:/sytest:ro \
    matrixdotorg/sytest-synapse:bookworm \
    tests/30rooms/12thirdpartyinvite.pl

Generic (from docs):

docker run --rm -it -v /path/to/synapse\:/src:ro -v /path/to/where/you/want/logs\:/logs \
    -v /path/to/code/sytest\:/sytest:ro matrixdotorg/sytest-synapse:bookworm

[MadLittleMods]

Closing in favor of #1425 which has the same branch name as the Synapse changes (element-hq/synapse#19723)