Move default realtime prompt into core (#17165)

- Adds a core-owned realtime backend prompt template and preparation
path.
- Makes omitted realtime start prompts use the core default, while null
or empty prompts intentionally send empty instructions.
- Covers the core realtime path and app-server v2 path with integration
coverage.

---------

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

codex-rs/core/tests/suite/compact_remote.rs

@@ -116,7 +116,7 @@ async fn start_remote_realtime_server() -> responses::WebSocketTestServer {
 async fn start_realtime_conversation(codex: &codex_core::CodexThread) -> Result<()> {
     codex
         .submit(Op::RealtimeConversationStart(ConversationStartParams {
-            prompt: "backend prompt".to_string(),
+            prompt: Some(Some("backend prompt".to_string())),
             session_id: None,
             transport: None,
         }))