Migrate the message mentioned groups to the generated UserGroupResponse - #6705

Goal

Parse the user groups mentioned on a message with the generated UserGroupResponse, retiring
DownstreamUserGroupDto and DownstreamUserGroupMemberDto.

Part of AND-1291

Implementation

• Point DownstreamMessageDto.mentioned_groups at the generated model. This was the last main-source
  in-edge on the hand-written pair: the user group endpoints already returned UserGroupResponse through
  their response wrappers, so UserGroupResponse.toDomain() already existed and was already exercised.
• Delete both hand-written DTOs with their two mappers, and the two mapper tests that duplicated the
  generated model's own.
• Drop the members block from the mentioned_groups fixture in MessageTestData.kt. Messages hydrate
  groups with commonpayloads.UserGroupResponse, which declares no Members field at all; the endpoints
  use a separate type that embeds it and adds members. The fixture described a payload the backend cannot
  send, and the generated UserGroupMember requires app_pk, so it stopped parsing.

Notes

The element type now matches the end state: the generated MessageResponse declares
mentioned_groups as List<UserGroupResponse>?. The container stays non-null here. thread_participants,
mentioned_groups and mentioned_roles share a documented explicit-null rejection on
DownstreamMessageDto, each with its own test, and loosening one of the three ahead of its siblings would
change null handling for a payload the wire never produces, since the Go tag is
json:"mentioned_groups,omitempty" on a slice. The three flip together when DownstreamMessageDto becomes
MessageResponse.

Three fields go from optional to required: created_at and updated_at on the group, which are plain
non-omitempty tags on the payload struct the wire serializes, so they always arrive.

No mutation sweep: this slice writes no mapper. UserGroupResponse.toDomain() is already on develop, so a
sweep would mutate code this PR does not touch.

Testing

Device-probed every path that carries mentioned_groups: the sendMessage response, the getMessage
re-read, and the message nested in a queryChannel response. All three returned the group with
created_at and updated_at populated and members empty, confirming the slim payload. The write path
still sends mentioned_group_ids and is unchanged by the swap.

Summary by CodeRabbit

• Bug Fixes

  • Updated mentioned-group message data to use the current user-group response format.
  • Improved handling of group metadata, including creation and update timestamps.
  • Removed obsolete group-member details from mentioned-group message data.

• Tests

  • Updated parsing, mapping, and test fixtures to reflect the current user-group response structure.