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/exec/src/lib.rs

@@ -976,6 +976,7 @@ async fn run_exec_session(args: ExecRunArgs) -> anyhow::Result<()> {
                         turn_trigger: None,
                         client_user_message_id: None,
                         input: items.into_iter().map(Into::into).collect(),
+                        tool_output: None,
                         responsesapi_client_metadata: None,
                         additional_context: None,
                         environments: None,