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/v2/realtime.rs

@@ -65,6 +65,9 @@ impl From<ThreadRealtimeAudioChunk> for CoreRealtimeAudioFrame {
 #[ts(export_to = "v2/")]
 pub struct ThreadRealtimeStartParams {
     pub thread_id: String,
+    /// Overrides the configured realtime model for this session only.
+    #[ts(optional = nullable)]
+    pub model: Option<String>,
     /// Selects text or audio output for the realtime session. Transport and voice stay
     /// independent so clients can choose how they connect separately from what the model emits.
     pub output_modality: RealtimeOutputModality,
@@ -80,6 +83,9 @@ pub struct ThreadRealtimeStartParams {
     pub realtime_session_id: Option<String>,
     #[ts(optional = nullable)]
     pub transport: Option<ThreadRealtimeStartTransport>,
+    /// Overrides the configured realtime protocol version for this session only.
+    #[ts(optional = nullable)]
+    pub version: Option<RealtimeConversationVersion>,
     #[ts(optional = nullable)]
     pub voice: Option<RealtimeVoice>,
 }