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/common.rs

@@ -1647,6 +1647,7 @@ mod tests {
     use codex_protocol::account::PlanType;
     use codex_protocol::models::BUILT_IN_PERMISSION_PROFILE_READ_ONLY;
     use codex_protocol::parse_command::ParsedCommand;
+    use codex_protocol::protocol::RealtimeConversationArchitecture;
     use codex_protocol::protocol::RealtimeConversationVersion;
     use codex_protocol::protocol::RealtimeOutputModality;
     use codex_protocol::protocol::RealtimeVoice;
@@ -3012,6 +3013,7 @@ mod tests {
         let request = ClientRequest::ThreadRealtimeStart {
             request_id: RequestId::Integer(9),
             params: v2::ThreadRealtimeStartParams {
+                architecture: Some(RealtimeConversationArchitecture::Avas),
                 thread_id: "thr_123".to_string(),
                 model: Some("realtime-treatment-model".to_string()),
                 output_modality: RealtimeOutputModality::Audio,
@@ -3027,6 +3029,7 @@ mod tests {
                 "method": "thread/realtime/start",
                 "id": 9,
                 "params": {
+                    "architecture": "avas",
                     "threadId": "thr_123",
                     "model": "realtime-treatment-model",
                     "outputModality": "audio",
@@ -3047,6 +3050,7 @@ mod tests {
         let default_prompt_request = ClientRequest::ThreadRealtimeStart {
             request_id: RequestId::Integer(9),
             params: v2::ThreadRealtimeStartParams {
+                architecture: None,
                 thread_id: "thr_123".to_string(),
                 model: None,
                 output_modality: RealtimeOutputModality::Audio,
@@ -3062,6 +3066,7 @@ mod tests {
                 "method": "thread/realtime/start",
                 "id": 9,
                 "params": {
+                    "architecture": null,
                     "threadId": "thr_123",
                     "model": null,
                     "outputModality": "audio",
@@ -3077,6 +3082,7 @@ mod tests {
         let null_prompt_request = ClientRequest::ThreadRealtimeStart {
             request_id: RequestId::Integer(9),
             params: v2::ThreadRealtimeStartParams {
+                architecture: None,
                 thread_id: "thr_123".to_string(),
                 model: None,
                 output_modality: RealtimeOutputModality::Audio,
@@ -3092,6 +3098,7 @@ mod tests {
                 "method": "thread/realtime/start",
                 "id": 9,
                 "params": {
+                    "architecture": null,
                     "threadId": "thr_123",
                     "model": null,
                     "outputModality": "audio",
@@ -3250,6 +3257,7 @@ mod tests {
         let request = ClientRequest::ThreadRealtimeStart {
             request_id: RequestId::Integer(1),
             params: v2::ThreadRealtimeStartParams {
+                architecture: None,
                 thread_id: "thr_123".to_string(),
                 model: None,
                 output_modality: RealtimeOutputModality::Audio,

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>,