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 {

coderd/database/queries/chats.sql

@@ -418,8 +418,8 @@ WHERE
         ELSE chats.archived = sqlc.narg('archived') :: boolean
     END
 ORDER BY
-    chats.created_at ASC,
-    chats.id ASC;
+    chats.created_at DESC,
+    chats.id DESC;
 
 -- name: InsertChat :one
 INSERT INTO chats (

coderd/exp_chats_test.go

@@ -1333,10 +1333,10 @@ func TestListChats(t *testing.T) {
 		}
 		require.Len(t, parent.Children, 2, "parent should embed 2 children")
 
-		// Children should be ordered by created_at ASC.
+		// Children are ordered by created_at DESC (newest first).
 		childIDs := []uuid.UUID{parent.Children[0].ID, parent.Children[1].ID}
-		require.Equal(t, child1.ID, childIDs[0])
-		require.Equal(t, child2.ID, childIDs[1])
+		require.Equal(t, child2.ID, childIDs[0])
+		require.Equal(t, child1.ID, childIDs[1])
 
 		// Verify each child has correct parent/root references.
 		for _, child := range parent.Children {