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/app-server-protocol/src/protocol/v2/realtime.rs

@@ -1,4 +1,5 @@
 use codex_protocol::protocol::RealtimeAudioFrame as CoreRealtimeAudioFrame;
+use codex_protocol::protocol::RealtimeConversationArchitecture;
 use codex_protocol::protocol::RealtimeConversationVersion;
 use codex_protocol::protocol::RealtimeOutputModality;
 use codex_protocol::protocol::RealtimeVoice;
@@ -65,6 +66,9 @@ impl From<ThreadRealtimeAudioChunk> for CoreRealtimeAudioFrame {
 #[ts(export_to = "v2/")]
 pub struct ThreadRealtimeStartParams {
     pub thread_id: String,
+    /// Overrides the configured realtime architecture for this session only.
+    #[ts(optional = nullable)]
+    pub architecture: Option<RealtimeConversationArchitecture>,
     /// Overrides the configured realtime model for this session only.
     #[ts(optional = nullable)]
     pub model: Option<String>,