Skip codex tool-call transcripts in realtime handoff

Stop copying Codex tool-call arguments into realtime handoff input transcripts so delegated text only comes from the tool-call transcription path.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/codex-api/tests/realtime_websocket_e2e.rs

@@ -450,7 +450,7 @@ async fn realtime_ws_e2e_realtime_v2_parser_emits_handoff_requested() {
         RealtimeEvent::HandoffRequested(RealtimeHandoffRequested {
             handoff_id: "call_123".to_string(),
             item_id: "item_123".to_string(),
-            input_transcript: "delegate now".to_string(),
+            input_transcript: String::new(),
             active_transcript: Vec::new(),
         })
     );