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.