Support standalone tool outputs in turn/start (#41002)

## What changed

- Add an optional `toolOutput` to `turn/start` for starting or steering a turn with a named function-call output instead of user input.
- Emit standalone outputs as `functionCallOutput` thread items, persist them in durable history, and include them in resumed threads.
- Validate that tool outputs have a name, are not combined with nonempty `input`, satisfy the text-size limit, and use supported image URLs.

## Testing

- Cover standalone tool-output notifications, model input, and resumed history in both legacy and paginated history modes.
- Cover the text-size limit for structured tool output.

GitOrigin-RevId: a1eeec7b9b7de67b1a1c5e829a54552b85ec118b

codex-rs/tui/src/thread_transcript.rs

@@ -281,6 +281,7 @@ fn fallback_transcript_cell(item: &ThreadItem) -> Option<PlainHistoryCell> {
         }
         ThreadItem::UserMessage { .. }
         | ThreadItem::AgentMessage { .. }
+        | ThreadItem::FunctionCallOutput { .. }
         | ThreadItem::Plan { .. }
         | ThreadItem::Reasoning { .. }
         | ThreadItem::Sleep(_) => return None,