MixinChatComponentLegacy

Chat capture for <=1.18.2, where chat has no system/player split: the 1-arg entry is the only one and sees everything. McpMixinPlugin applies this ONLY where no player-chat entry exists, so it never coexists with MixinChatComponentFunnel — which would double every system message, the 1-arg being a pure delegate into the funnel from 1.19 on.