realtime: add AVAS architecture override (#27720)

## Summary

Adds a `RealtimeConversationArchitecture` option for realtime
conversation startup, with `realtimeapi` as the default and `avas` as an
opt-in architecture.

The AVAS path is limited to realtime v1 conversational WebRTC starts,
and WebRTC call creation appends `intent=quicksilver&architecture=avas`
to `/v1/realtime/calls`. The existing sideband websocket still joins by
`call_id`.

This also exposes the per-session architecture override through
app-server v2 `thread/realtime/start` params and updates the config
schema for `[realtime].architecture`.

## Validation

- `just fmt`
- `just write-config-schema`
- `just test -p codex-api sends_avas_session_call_query_params`
- `just test -p codex-core -E
'test(~conversation_webrtc_start_uses_avas_architecture_query)'`
- `just test -p codex-core -E 'test(realtime_loads_from_config_toml)'`
- `just test -p codex-app-server-protocol -E
'test(~serialize_thread_realtime_start) |
test(generated_ts_optional_nullable_fields_only_in_params)'`
- `just test -p codex-app-server -E
'test(realtime_webrtc_start_emits_sdp_notification)'`

codex-rs/protocol/src/protocol.rs

@@ -185,6 +185,8 @@ pub struct McpServerRefreshConfig {
 
 #[derive(Debug, Clone, PartialEq)]
 pub struct ConversationStartParams {
+    /// Overrides the configured realtime architecture for this session only.
+    pub architecture: Option<RealtimeConversationArchitecture>,
     /// Overrides the configured realtime model for this session only.
     pub model: Option<String>,
     /// Selects whether the realtime session should produce text or audio output.
@@ -1495,6 +1497,15 @@ pub enum RealtimeConversationVersion {
     V2,
 }
 
+#[derive(Debug, Clone, Copy, Default, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "snake_case")]
+pub enum RealtimeConversationArchitecture {
+    #[default]
+    #[serde(rename = "realtimeapi")]
+    RealtimeApi,
+    Avas,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize, PartialEq, JsonSchema, TS)]
 pub struct RealtimeConversationStartedEvent {
     pub realtime_session_id: Option<String>,