realtime: add AVAS architecture override

[bakks]

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)'

[bakks]

Reviewer context after smoke testing:

• Prod AVAS smoke succeeded against prod transceiver using session rtc_u7_DpoFrztZS62whWbwp64OV.
• Transceiver logs showed POST /v1/realtime/calls?intent=quicksilver&architecture=avas returning 201, output=realtimeapi-avas, client_model=bidi-garlic-latest-api-ev3, Connected to voice app server, and Received AV App init response.
• Sideband joined the same call with GET /v1/realtime?intent=quicksilver&call_id=rtc_u7_DpoFrztZS62whWbwp64OV returning 200.
• Follow-up cleanup removed the temporary experimental_realtime_webrtc_call_path override; WebRTC call creation now always uses the normal realtime/calls path, so future local transceiver smoke should expose that path rather than relying on a client path override.

[guinness-oai]

lgtm!

[aibrahim-oai]

can we add a todo to remove the branching?