Sessions: grow realtime sessions into a multi-provider framework (OpenAI-only today)

[AbirAbbas]

Umbrella tracking issue from #654.

#654 ships a deliberately provider-agnostic session DX — provider is an explicit, required knob and the transport matrix is multi-provider — but OpenAI WebRTC is the only fully-wired realtime path today. realtime-offer hard-gates to provider=openai+transport=webrtc and posts to api.openai.com/v1/realtime/calls; the declared openrouter: audio_turns pair has no handler.

This issue tracks growing sessions into genuinely multi-provider realtime ingress, so AgentField is the provider boundary for more than just OpenAI.

Candidate providers / transports

• OpenRouter (voice-capable models) — tracked in Sessions: implement real OpenRouter provider support + voice-capable models (audio_turns is currently a no-op) #665.
• Azure OpenAI Realtime — near drop-in; Azure documents the same SDP-proxy pattern (/openai/v1/realtime/calls, ephemeral via /openai/v1/realtime/client_secrets). Mostly an endpoint + auth swap in createOpenAIRealtimeCall.
• Google Gemini Live — different protocol (WebSocket bidi streaming, not WebRTC/SDP); needs a dedicated control-plane adapter.
• Deepgram Voice Agent / others — evaluate transport + tool-calling fit.

Design principles to hold across providers

• Provider key stays server-side (the "client never holds the provider key" boundary from Add AgentField realtime sessions for WebRTC voice ingress #654).
• provider/transport stay explicit — no inference, no silent switching (existing validation philosophy).
• Tool calls route through execute/async with X-Session-ID so every provider lands in the same workflow DAG.
• The transport matrix only advertises pairs that have a real handler (no no-op entries — ties into Sessions: implement websocket transport + wire RealtimeSession.input()/say() live loop (currently scaffolding) #664).

Acceptance criteria (behavior)

• At least one non-OpenAI provider produces a working realtime session end-to-end through the control plane, key server-side.
• Adding a provider is a documented extension point (handler + matrix entry + session-config mapping), not a fork of the OpenAI path.

Related: #662 (sideband tools), #663 (VAD/turn-detection config), #664 (websocket transport + live loop), #665 (OpenRouter).

[AbirAbbas]

Triage

Classification: feature ✓
Suggested labels: enhancement

Likely affected files:

• ./reasoners/models.py (matched: models, one)
• ./reasoners/repo_config.py (matched: one, config)
• ./reasoners/webhook_router.py (matched: route, one)
• ./reasoners/init.py (matched: one)
• ./reasoners/conversational.py (matched: one)

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Triage by github-buddy