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/core/src/client.rs

@@ -77,6 +77,7 @@ use codex_protocol::models::ResponseItem;
 use codex_protocol::openai_models::ModelInfo;
 use codex_protocol::openai_models::ReasoningEffort as ReasoningEffortConfig;
 use codex_protocol::protocol::InternalSessionSource;
+use codex_protocol::protocol::RealtimeConversationArchitecture;
 use codex_protocol::protocol::SessionSource;
 use codex_protocol::protocol::W3cTraceContext;
 use codex_rollout_trace::CompactionTraceContext;
@@ -522,7 +523,9 @@ impl ModelClient {
         &self,
         sdp: String,
         session_config: ApiRealtimeSessionConfig,
+        architecture: RealtimeConversationArchitecture,
         mut extra_headers: ApiHeaderMap,
+        api_provider_override: Option<ApiProvider>,
     ) -> Result<RealtimeWebrtcCallStart> {
         // Create the media call over HTTP first, then retain matching auth so realtime can attach
         // the server-side control WebSocket to the call id from that HTTP response.
@@ -535,11 +538,16 @@ impl ModelClient {
             client_setup.api_auth.as_ref(),
         ));
         let transport = ReqwestTransport::new(build_reqwest_client());
-        let response =
-            ApiRealtimeCallClient::new(transport, client_setup.api_provider, client_setup.api_auth)
-                .create_with_session_and_headers(sdp, session_config, extra_headers)
-                .await
-                .map_err(map_api_error)?;
+        let api_provider = api_provider_override.unwrap_or(client_setup.api_provider);
+        let response = ApiRealtimeCallClient::new(transport, api_provider, client_setup.api_auth)
+            .create_with_session_architecture_and_headers(
+                sdp,
+                session_config,
+                architecture,
+                extra_headers,
+            )
+            .await
+            .map_err(map_api_error)?;
         Ok(RealtimeWebrtcCallStart {
             sdp: response.sdp,
             call_id: response.call_id,