refactor(chat): remove dead syntaxTheme plumbing

The Prism syntaxTheme prop is no longer read after code highlighting moved to
the Shiki worker. Remove the now-dead prop threading and its source.

- Drop syntaxTheme from interfaces, destructures, prop passes, and React.memo
  comparators across ChatMessage, MessageBody, ProgressiveGroup, ToolPart,
  TurnActivity, ToolOutputDialog, and ChatInput.
- Drop the unused _syntaxTheme param from renderWebSearchOutput.
- Remove the dead generateSyntaxTheme usages (ChatMessage memo, PlanView
  unassigned memo) and delete the now-unimported syntaxThemeGenerator module.

packages/ui/src/components/chat/message/toolRenderers.tsx

@@ -510,7 +510,7 @@ export const renderTodoOutput = (
     }
 };
 
-export const renderWebSearchOutput = (output: string, _syntaxTheme: { [key: string]: React.CSSProperties }, options?: { unstyled?: boolean }) => {
+export const renderWebSearchOutput = (output: string, options?: { unstyled?: boolean }) => {
     try {
         return (
             <div