[upstream #16961] PR 16961: Include the ask-user part in the stored content union #320

Codex finding on LibreChat-AI#16961: LibreChat-AI#16961 (comment)

The client writes a synthetic ask_user_question part into TMessage.content to render an ask-user pause, but TMessageContentParts does not include it. This predates LibreChat-AI#16961: client/src/utils/approval.ts documents the gap and reads the type through a cast in isAskUserQuestionPart. LibreChat-AI#16961 shares the shape as AskUserQuestionContentPart in packages/data-provider/src/parts.ts, and fromUIPart returns it as TMessageContentParts through a cast.

Runtime behavior is correct (the part round-trips losslessly), so this is type debt: add the part to the stored content union (or a dedicated TMessage.content element type), then drop the casts in fromUIPart, isAskUserQuestionPart and the specs. Widening the union reaches every exhaustive switch over content types in the client, so it is its own change.