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/protocol/src/protocol.rs

@@ -183,11 +183,15 @@ pub struct McpServerRefreshConfig {
 
 #[derive(Debug, Clone, PartialEq)]
 pub struct ConversationStartParams {
+    /// Overrides the configured realtime model for this session only.
+    pub model: Option<String>,
     /// Selects whether the realtime session should produce text or audio output.
     pub output_modality: RealtimeOutputModality,
     pub prompt: Option<Option<String>>,
     pub realtime_session_id: Option<String>,
     pub transport: Option<ConversationStartTransport>,
+    /// Overrides the configured realtime protocol version for this session only.
+    pub version: Option<RealtimeConversationVersion>,
     pub voice: Option<RealtimeVoice>,
 }