Migrate the message moderation and reaction groups to the generated models - #6685

Goal

Adopt the generated ModerationV2Response and ReactionGroupResponse for the last two nested leaves of
DownstreamMessageDto.

Part of AND-1291

Implementation

• Point DownstreamMessageDto.moderation and reaction_groups at the generated models and follow the
  field renames in both mappers. DownstreamModerationDto and DownstreamReactionGroupDto go away;
  ReactionDtos.kt becomes DownstreamReactionDto.kt, since that is all it still holds.
• Vendor ModerationV2Response, ReactionGroupResponse and ReactionGroupUserResponse, which the former
  references as a field type.

Notes

Neither swap changes strictness. ReactionGroupResponse requires the same four fields the hand-written DTO
declared non-null, and ModerationV2Response requires action and original_text and leaves the rest
nullable, exactly as before.

Neither mapping can diverge, because of how the backend tags the struct. action and original_text are
plain tags and always serialized, and every other field is omitempty on a non-pointer type, so it is
either a real value or absent and can never arrive as JSON null. That is the only input that would tell
the old "nullable, no default" fields apart from the generated "nullable with default" ones, so an absent
key produces the same Moderation through both: .orEmpty() for the harm lists, null for the two strings,
?: false for platform_circumvented.

ReactionGroupResponse.latest_reactions_by has no home on the domain and is read then dropped, like the
config fields in
AND-1498.
blocklists_matched is also parsed and dropped, but it is a different problem: the backend deprecates the
singular blocklist_matched that Moderation.blocklistMatched maps, in favour of exactly that plural.
Tracked as
AND-1502.

latest_reactions_by is the one key where the two paths now differ: the generated model declares it, so
it is parsed and its entries are validated, while ReactionGroupAdapter still skips it. Neither reaches
the domain. It also makes NullCollectionsAsEmptyFactory match ReactionGroupResponse, which is
load-bearing rather than incidental: the field is a non-null list against a plain non-omitempty Go tag,
so a message that predates the latest_reactions_by backfill sends an explicit null that would otherwise
throw. Both cases are pinned by tests.

Testing

• ModerationParsingTest and ReactionGroupParsingTest retargeted to the generated models, and
  DomainMappingTest covers both mappings.
• Mutation sweep over both mappers: ReactionGroupResponse.toDomain has all five assignments caught, and
  every field of ModerationV2Response.toDomain is compile-guaranteed, since Moderation defaults none of
  them.
• Device-probed reaction_groups on the wire across the four shapes that carry it: getMessage,
  queryChannels, reaction.new and reaction.deleted. Every ReactionGroup field asserted, with two
  reaction types at different scores so count and sumScore cannot be confused.
• ReactionGroupParsingTest covers a group carrying latest_reactions_by and one carrying an explicit
  null, asserting both paths still produce an identical ReactionGroup.
• moderation is covered by unit tests only. The server sets it only when moderation blocks a message, so
  reaching it needs a channel type whose automod behaviour blocks or bounces rather than flags, and that is
  server-side app config the demo app does not have.

Summary by CodeRabbit

• Bug Fixes

  • Improved message handling for moderation results and reaction-group data.
  • Updated parsing to support the latest response field formats, including optional moderation details and reaction-group users.

• Tests

  • Expanded validation for complete, partial, null, and invalid moderation and reaction-group responses.
  • Improved parsing error checks to identify the affected response fields.