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/app-server-protocol/src/protocol/common.rs

@@ -2942,10 +2942,12 @@ mod tests {
             request_id: RequestId::Integer(9),
             params: v2::ThreadRealtimeStartParams {
                 thread_id: "thr_123".to_string(),
+                model: Some("realtime-treatment-model".to_string()),
                 output_modality: RealtimeOutputModality::Audio,
                 prompt: Some(Some("You are on a call".to_string())),
                 realtime_session_id: Some("sess_456".to_string()),
                 transport: None,
+                version: Some(RealtimeConversationVersion::V1),
                 voice: Some(RealtimeVoice::Marin),
             },
         };
@@ -2955,10 +2957,12 @@ mod tests {
                 "id": 9,
                 "params": {
                     "threadId": "thr_123",
+                    "model": "realtime-treatment-model",
                     "outputModality": "audio",
                     "prompt": "You are on a call",
                     "realtimeSessionId": "sess_456",
                     "transport": null,
+                    "version": "v1",
                     "voice": "marin"
                 }
             }),
@@ -2973,10 +2977,12 @@ mod tests {
             request_id: RequestId::Integer(9),
             params: v2::ThreadRealtimeStartParams {
                 thread_id: "thr_123".to_string(),
+                model: None,
                 output_modality: RealtimeOutputModality::Audio,
                 prompt: None,
                 realtime_session_id: None,
                 transport: None,
+                version: None,
                 voice: None,
             },
         };
@@ -2986,9 +2992,11 @@ mod tests {
                 "id": 9,
                 "params": {
                     "threadId": "thr_123",
+                    "model": null,
                     "outputModality": "audio",
                     "realtimeSessionId": null,
                     "transport": null,
+                    "version": null,
                     "voice": null
                 }
             }),
@@ -2999,10 +3007,12 @@ mod tests {
             request_id: RequestId::Integer(9),
             params: v2::ThreadRealtimeStartParams {
                 thread_id: "thr_123".to_string(),
+                model: None,
                 output_modality: RealtimeOutputModality::Audio,
                 prompt: Some(None),
                 realtime_session_id: None,
                 transport: None,
+                version: None,
                 voice: None,
             },
         };
@@ -3012,10 +3022,12 @@ mod tests {
                 "id": 9,
                 "params": {
                     "threadId": "thr_123",
+                    "model": null,
                     "outputModality": "audio",
                     "prompt": null,
                     "realtimeSessionId": null,
                     "transport": null,
+                    "version": null,
                     "voice": null
                 }
             }),
@@ -3168,10 +3180,12 @@ mod tests {
             request_id: RequestId::Integer(1),
             params: v2::ThreadRealtimeStartParams {
                 thread_id: "thr_123".to_string(),
+                model: None,
                 output_modality: RealtimeOutputModality::Audio,
                 prompt: Some(Some("You are on a call".to_string())),
                 realtime_session_id: None,
                 transport: None,
+                version: None,
                 voice: None,
             },
         };