ChatBox.SendMessage reads bytes from ValidateMessage so Encoding.UTF8.GetBytes
runs once per send. ValidateMessage takes an injectable sanitiser so xUnit can
exercise the length-equality gate without ClientStructs game memory.
CompactInputSubmitter and CompactInputHistoryNavigator lift the deterministic
parts of ChatInputBar's pop-out submit and history-up/down callback into POCO
helpers under HellionChat/_Helpers/. The ImGui buffer splice
(DeleteChars/InsertChars) stays at the call site because it needs the live
callback data.
Behavior is identical to the previous inline implementation; tests in the
local Build Suite repo pin the contracts.
JonKazama-Hellion
1c354d18bb