Add the thread participant fields sent by the real backend

[gpunto]

🔗 Issue Links

Needed by GetStream/stream-chat-android#6620, which migrates Android's thread participants to the OpenAPI-generated ThreadParticipant model.

/threads is synthesised in src/helpers/threads.rb rather than recorded by sync.rb (threads have to be derived from the messages each test creates), and the synthesised thread_participants had drifted from the real payload: it carried only user_id and user.

The real backend also sends app_pk, channel_cid, thread_id, created_at, last_read_at, last_thread_message_at and custom — all of which the OpenAPI spec marks required on ThreadParticipant. Clients generated from the spec therefore fail to decode the mock response.

This adds those fields, derived per participant from that user's own replies. The change is purely additive — no existing field is renamed or removed — so SDKs that don't read them are unaffected.

🧪 Testing Notes

Please verify the E2E test runs:

• iOS
  • run 31161573028
• Android
  • run 31101798706
• Flutter
  • run 31161577192