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/src/request_processors/turn_processor.rs

@@ -934,6 +934,7 @@ impl TurnRequestProcessor {
             request_id,
             thread.as_ref(),
             Op::RealtimeConversationStart(ConversationStartParams {
+                model: params.model,
                 output_modality: params.output_modality,
                 prompt: params.prompt,
                 realtime_session_id: params.realtime_session_id,
@@ -945,6 +946,7 @@ impl TurnRequestProcessor {
                         ConversationStartTransport::Webrtc { sdp }
                     }
                 }),
+                version: params.version,
                 voice: params.voice,
             }),
         )