fix: sort child chats newest-first and prepend on creation (#24524)

GetChildChatsByParentIDs sorted created_at ASC, but the cache
helper appended new children to the end. On refetch the API and
cache agreed on oldest-first, putting the just-created child at
the bottom. Users expect newest first, matching the root-chat
sidebar convention.

- SQL: change child sort to created_at DESC, id DESC.
- Cache: prepend instead of append in addChildToParentInCache
  (renamed from appendChildToParentInCache to avoid leaking
  position semantics).
- Test: update ordering assertion to expect newest-first.

Refs #24404

coderd/database/queries.sql.go

@@ -6881,8 +6881,8 @@ WHERE
         ELSE chats.archived = $2 :: boolean
     END
 ORDER BY
-    chats.created_at ASC,
-    chats.id ASC
+    chats.created_at DESC,
+    chats.id DESC
 `
 
 type GetChildChatsByParentIDsParams struct {