Add per-session realtime model and version overrides (#24999)

## Why

Clients need to select a realtime session configuration for an
individual start without rewriting persisted configuration or restarting
the app-server process.

## What Changed

- Add optional `model` and `version` fields to `thread/realtime/start`
- Forward those optional values through the realtime start operation and
apply them only for that session
- Preserve existing configured/default behavior when the new fields are
omitted
- Update generated protocol schema and app-server documentation

## Validation

- Added/updated protocol serialization coverage for the new optional
request fields
- Added focused core coverage for a session override taking precedence
over configured realtime selection
- Added focused app-server coverage that a request override reaches the
realtime WebSocket handshake

codex-rs/core/src/realtime_conversation.rs

@@ -625,12 +625,14 @@ async fn prepare_realtime_start(
     if let Some(realtime_ws_base_url) = &config.experimental_realtime_ws_base_url {
         api_provider.base_url = realtime_ws_base_url.clone();
     }
-    let version = config.realtime.version;
+    let version = params.version.unwrap_or(config.realtime.version);
     let session_config = build_realtime_session_config(
         sess,
+        params.model,
         params.prompt,
         params.realtime_session_id,
         params.output_modality,
+        version,
         params.voice,
     )
     .await?;
@@ -664,9 +666,11 @@ async fn prepare_realtime_start(
 
 pub(crate) async fn build_realtime_session_config(
     sess: &Arc<Session>,
+    model: Option<String>,
     prompt: Option<Option<String>>,
     realtime_session_id: Option<String>,
     output_modality: RealtimeOutputModality,
+    version: RealtimeWsVersion,
     voice: Option<RealtimeVoice>,
 ) -> CodexResult<RealtimeSessionConfig> {
     let config = sess.get_config().await;
@@ -689,18 +693,15 @@ pub(crate) async fn build_realtime_session_config(
         (false, false) => format!("{prompt}\n\n{startup_context}"),
     };
     let model = Some(
-        config
-            .experimental_realtime_ws_model
-            .clone()
+        model
+            .or_else(|| config.experimental_realtime_ws_model.clone())
             .unwrap_or_else(|| DEFAULT_REALTIME_MODEL.to_string()),
     );
-    let event_parser = match config.realtime.version {
+    let event_parser = match version {
         RealtimeWsVersion::V1 => RealtimeEventParser::V1,
         RealtimeWsVersion::V2 => RealtimeEventParser::RealtimeV2,
     };
-    if config.realtime.version == RealtimeWsVersion::V1
-        && matches!(output_modality, RealtimeOutputModality::Text)
-    {
+    if version == RealtimeWsVersion::V1 && matches!(output_modality, RealtimeOutputModality::Text) {
         return Err(CodexErr::InvalidRequest(
             "text realtime output modality requires realtime v2".to_string(),
         ));
@@ -711,8 +712,8 @@ pub(crate) async fn build_realtime_session_config(
     };
     let voice = voice
         .or(config.realtime.voice)
-        .unwrap_or_else(|| default_realtime_voice(config.realtime.version));
-    validate_realtime_voice(config.realtime.version, voice)?;
+        .unwrap_or_else(|| default_realtime_voice(version));
+    validate_realtime_voice(version, voice)?;
     Ok(RealtimeSessionConfig {
         instructions: prompt,
         model,