Event object no longer has "membership" key

First reported at https://github.com/zulip/python-zulip-api/issues/612. For example, code like https://github.com/matrix-org/matrix-python-sdk/blob/a469d531b6879c5687718937a0bfcb443155ce43/samples/SimpleChatClient.py#L26 no longer works.

The list of keys of the event object as of now:
`['unsigned', 'origin_server_ts', 'type', 'room_id', 'content', 'sender', 'event_id']`.

Start by tracing how the SDK constructs the event object and compare its current keys with the membership access in samples/SimpleChatClient.py at line 26. Confirm the intended event shape from the linked report, restore the missing membership data if appropriate, and verify that the sample no longer fails.

Written by the indexing model from the issue text.