fix(chat): preview the last message with content after a delete

[bmc08gt]

Deleting the newest message in a DM left the Chats row reading "Message deleted" with the unread splat still beside it. iOS falls back to the newest message that still has content, and the splat clears with it. This brings Android to that behaviour.

How it works

Both symptoms come from one field. The feed's ChatMetadata.lastMessage is what the row previews and what its unread check compares against the READ pointer, so FeedSyncDelegate.buildFeedFromDb reading getLatestVisible instead of getLatest fixes both at once.

Tombstones are filtered on a new chat_messages.is_deleted column rather than matching the serialized discriminator in content_json. There is precedent for the LIKE approach in hasEverTipped, but here a text message quoting "type":"deleted" would be misread as a tombstone. The mapper writes the flag from the domain content, so only rows cached before the column existed need repair — DB 31 -> 32 adds the column and backfills them, and that is the one place the LIKE survives, where a false positive is bounded and self-heals on the next server sync.

markAsRead needed a companion change. It previously preferred the feed's lastMessage?.messageId, which no longer identifies the newest row; left alone it would have parked the READ pointer below the deleted message and left the chat unread forever. It now prefers messageDataSource.getLatestMessageId, tombstones included. This is the same split iOS draws between latestMessage (filters kind != 2) and newestMessage / newestMessageID.

ChatSummaryMapping.formatPreview returns null for a tombstone, which is now only reachable when every message in a chat is deleted. The label_chat_preview_deletedMessage string is deleted with its last caller; the in-transcript bubble copy (label_messageDeleted*) is untouched.

Tests

Nine new tests across the three touched layers:

• FeedDeletedMessageTest covers the two user-visible behaviours, the case that must not change (a fallback message that was itself never read keeps its splat), and the all-deleted chat.
• ChatMessageDaoTest covers the getLatest / getLatestVisible split and runs the migration backfill SQL against real serialized rows, pinning the matched string to what the converters actually write.
• One ChatEntityMapperTest case pins is_deleted to the message content.

Note

The feed still updates on server confirmation rather than optimistically — PendingMutation reaches the transcript only, not the Chats list. That matches iOS, so it is left as is; your own delete refreshes the open conversation immediately and the list follows when the event lands.