fix MSC3912-relation-based-redaction for room versions > 10

[jason-famedly]

Room version 11 moved the redacts key for redaction events from top-level to inside the contents object of an Event. Incorporate the change needed to maintain this MSC remains operational.

This fixes some MSC3912 behavior when used with room versions > 10.

Needed to pass room version 11 & 12 unit tests. 3 fail without this

Test results before

tests.rest.client.test_redactions
  RedactionsTestCase
    test_redact_all_relations ...                                        [FAIL]
    test_redact_create_event ...                                           [OK]
    test_redact_event_as_moderator ...                                     [OK]
    test_redact_event_as_moderator_ratelimit ...                           [OK]
    test_redact_event_as_normal ...                                        [OK]
    test_redact_nonexistent_event ...                                      [OK]
    test_redact_relations_no_perms ...                                   [FAIL]
    test_redact_relations_txn_id_reuse ...                                 [OK]
    test_redact_relations_with_types ...                                 [FAIL]
    test_redaction_content_0 ...                                           [OK]
    test_redaction_content_1 ...                                           [OK]
    test_redaction_content_2 ...                                           [OK]

===============================================================================
[FAIL]
Traceback (most recent call last):
  File "/home/jason/dev/famedly/synapse-upstreaming/tests/rest/client/test_redactions.py", line 414, in test_redact_all_relations
    self.assertIn("redacted_because", event_dict, event_dict)
  File "/home/jason/.cache/pypoetry/virtualenvs/matrix-synapse-vUbQZmJ1-py3.13/lib/python3.13/site-packages/twisted/trial/_synctest.py", line 509, in assertIn
    raise self.failureException(msg or f"{containee!r} not in {container!r}")
twisted.trial.unittest.FailTest: {'age': 500, 'content': {'body': ' * hello world', 'm.new_content': {'body': 'hello world', 'msgtype': 'm.text'}, 'm.relates_to': {'event_id': '$e4aURQ2oCjklgu0CFv6rq7VXBKTizeXDvw3sq69oLbI', 'rel_type': 'm.replace'}, 'msgtype': 'm.text'}, 'event_id': '$DYlYk9M5ayjHEErwMBgm-x0kgLA9-MuJYXE7lhn31Cg', 'origin_server_ts': 1101, 'room_id': '!qoN5VJOacdt4kvnXFBjqdo1isnshVBVA7-aAYRL79fs', 'sender': '@user1:test', 'type': 'm.room.message', 'unsigned': {'age': 500, 'membership': 'join', 'transaction_id': 'm1778701690.8760948'}, 'user_id': '@user1:test'}

tests.rest.client.test_redactions.RedactionsTestCase.test_redact_all_relations
===============================================================================
[FAIL]
Traceback (most recent call last):
  File "/home/jason/dev/famedly/synapse-upstreaming/tests/rest/client/test_redactions.py", line 503, in test_redact_relations_no_perms
    self.assertIn("redacted_because", event_dict, event_dict)
  File "/home/jason/.cache/pypoetry/virtualenvs/matrix-synapse-vUbQZmJ1-py3.13/lib/python3.13/site-packages/twisted/trial/_synctest.py", line 509, in assertIn
    raise self.failureException(msg or f"{containee!r} not in {container!r}")
twisted.trial.unittest.FailTest: {'age': 300, 'content': {'body': 'message 2', 'm.relates_to': {'event_id': '$9cTuhYe4gRPI_WdH4ISZUAoo_3MfSvDkjy4zVtfkLzs', 'rel_type': 'm.thread'}, 'msgtype': 'm.text'}, 'event_id': '$Eg0Rlk1_KVNVZPuhutegwaE1NkRLmuuQJ4Zdm9_ywRE', 'origin_server_ts': 1201, 'room_id': '!qoN5VJOacdt4kvnXFBjqdo1isnshVBVA7-aAYRL79fs', 'sender': '@otheruser:test', 'type': 'm.room.message', 'unsigned': {'age': 300, 'membership': 'join', 'transaction_id': 'm1778701693.5619848'}, 'user_id': '@otheruser:test'}

tests.rest.client.test_redactions.RedactionsTestCase.test_redact_relations_no_perms
===============================================================================
[FAIL]
Traceback (most recent call last):
  File "/home/jason/dev/famedly/synapse-upstreaming/tests/rest/client/test_redactions.py", line 316, in test_redact_relations_with_types
    self.assertIn("redacted_because", event_dict, event_dict)
  File "/home/jason/.cache/pypoetry/virtualenvs/matrix-synapse-vUbQZmJ1-py3.13/lib/python3.13/site-packages/twisted/trial/_synctest.py", line 509, in assertIn
    raise self.failureException(msg or f"{containee!r} not in {container!r}")
twisted.trial.unittest.FailTest: {'age': 500, 'content': {'body': ' * hello world', 'm.new_content': {'body': 'hello world', 'msgtype': 'm.text'}, 'm.relates_to': {'event_id': '$e4aURQ2oCjklgu0CFv6rq7VXBKTizeXDvw3sq69oLbI', 'rel_type': 'm.replace'}, 'msgtype': 'm.text'}, 'event_id': '$DYlYk9M5ayjHEErwMBgm-x0kgLA9-MuJYXE7lhn31Cg', 'origin_server_ts': 1101, 'room_id': '!qoN5VJOacdt4kvnXFBjqdo1isnshVBVA7-aAYRL79fs', 'sender': '@user1:test', 'type': 'm.room.message', 'unsigned': {'age': 500, 'membership': 'join', 'transaction_id': 'm1778701694.2228405'}, 'user_id': '@user1:test'}

tests.rest.client.test_redactions.RedactionsTestCase.test_redact_relations_with_types

A bit more context:
By using the raw initial EventBase from the original redaction request, this included the redacts key in contents for the original targeted event that needed redaction. This was not being changed or overwritten by the new redaction event that was being created to handle a related event, instead a new top-level redacts key was produced pointing at the correct targeted event ID. After construction of the EventBase for this new event(in create_and_send_nonmember_event()) the new EventBases redact property first reads the key in the content object and returns, which ignores the top-level redacts. So this mechanism was trying to redact the wrong(and already redacted!) original event per the request.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
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• Code style is correct (run the linters)