Always use AVAS for realtime WebRTC calls (#28856)

## Summary

- Remove the realtime `architecture` selector from core protocol,
app-server protocol, config parsing, generated schemas, and callers.
- Always create WebRTC realtime calls with the AVAS query params:
`intent=quicksilver&architecture=avas`.
- Keep direct websocket realtime behavior on the existing config/default
path, while WebRTC starts without an explicit version now default to
realtime v1 because AVAS requires v1.

## Notes

- WebRTC realtime now means AVAS. If a caller explicitly asks to start
WebRTC with realtime v2, Codex rejects that request because the AVAS
WebRTC path only supports realtime v1. Websocket realtime is separate
and can still use realtime v2.
- The old `[realtime] architecture = "realtimeapi" | "avas"` config knob
is removed. Local configs that still set it will need to delete that
line.
- Some app-server tests that were only trying to exercise realtime v2
protocol behavior now use websocket transport, because WebRTC is
intentionally locked to AVAS/v1. Separate WebRTC tests cover the AVAS
query params, v1 startup, SDP flow, and sideband join.

## Validation

- Merged fresh `origin/main` at `83e6a786a2`.
- `just fmt`
- `just write-config-schema`
- `just write-app-server-schema`
- `git diff --check`
- `just test -p codex-api -p codex-core -p codex-app-server-protocol -p
codex-app-server realtime` (176 passed)
- `just test -p codex-protocol -p codex-config` (413 passed)

codex-rs/config/src/config_toml.rs

@@ -587,14 +587,12 @@ pub enum RealtimeTransport {
     Websocket,
 }
 
-pub use codex_protocol::protocol::RealtimeConversationArchitecture as RealtimeArchitecture;
 pub use codex_protocol::protocol::RealtimeConversationVersion as RealtimeWsVersion;
 pub use codex_protocol::protocol::RealtimeVoice;
 
 #[derive(Serialize, Deserialize, Debug, Clone, Default, PartialEq, Eq, JsonSchema)]
 #[schemars(deny_unknown_fields)]
 pub struct RealtimeConfig {
-    pub architecture: RealtimeArchitecture,
     pub version: RealtimeWsVersion,
     #[serde(rename = "type")]
     pub session_type: RealtimeWsMode,
@@ -605,7 +603,6 @@ pub struct RealtimeConfig {
 #[derive(Serialize, Deserialize, Debug, Clone, Default, PartialEq, Eq, JsonSchema)]
 #[schemars(deny_unknown_fields)]
 pub struct RealtimeToml {
-    pub architecture: Option<RealtimeArchitecture>,
     pub version: Option<RealtimeWsVersion>,
     #[serde(rename = "type")]
     pub session_type: Option<RealtimeWsMode>,