[codex] Update realtime V2 VAD silence delay and 1.5 prompt (#18092)

## Summary

- set the realtime v2 server VAD silence delay to 500ms
- update the default realtime 1.5 backend prompt to the v4 text
- keep the session payload and prompt rendering tests aligned with those
changes

## Why

- the VAD change gives the voice path a longer pause before ending the
user's turn
- the prompt change makes the default bundled realtime prompt match the
current v4 content

## Validation

- `cargo +1.93.0 test -p codex-core realtime_prompt --manifest-path
/tmp/codex-realtime-v2-vad-prompt-v4/codex-rs/Cargo.toml`
- `CARGO_TARGET_DIR=/tmp/codex-pr-v4-target cargo +1.93.0 test -p
codex-api
realtime_v2_session_update_includes_background_agent_tool_and_handoff_output_item
--manifest-path
/tmp/codex-realtime-v2-vad-prompt-v4/codex-rs/Cargo.toml`
- `CARGO_TARGET_DIR=/tmp/codex-pr-v4-target cargo +1.93.0 test -p
codex-app-server --test all
'suite::v2::realtime_conversation::realtime_webrtc_start_emits_sdp_notification'
--manifest-path /tmp/codex-realtime-v2-vad-prompt-v4/codex-rs/Cargo.toml
-- --exact`

codex-rs/codex-api/src/endpoint/realtime_websocket/methods.rs

@@ -1588,6 +1588,7 @@ mod tests {
                     "type": "server_vad",
                     "interrupt_response": true,
                     "create_response": true,
+                    "silence_duration_ms": 500,
                 })
             );
             assert_eq!(