Sessions: implement real OpenRouter provider support + voice-capable models (audio_turns is currently a no-op)

[AbirAbbas]

Follow-up from #654 / discussion.

The session transport matrix already declares openrouter: {audio_turns}, but audio_turns has no handler anywhere in the control plane — realtime-offer hard-gates to provider=openai+transport=webrtc and posts to api.openai.com/v1/realtime/calls. So today the matrix advertises an OpenRouter capability that does nothing.

This issue tracks making OpenRouter a real, working session provider, including its voice-capable models.

Why

• Add AgentField realtime sessions for WebRTC voice ingress #654 ships a deliberately provider-agnostic session DX (provider is an explicit, required knob) but only OpenAI is actually wired. OpenRouter is the declared second provider; it should either work or not be advertised.
• OpenRouter brokers many voice/realtime-capable models; supporting it makes AgentField sessions genuinely multi-provider rather than OpenAI-locked.

Scope / open design questions

• Decide what audio_turns actually is (turn-based audio request/response) vs. whether OpenRouter exposes a realtime/WebRTC path, and implement the matching control-plane handler — not just a matrix entry.
• Wire the provider boundary (OpenRouter API key server-side, same "client never holds the key" model as OpenAI).
• Map session config (model, voice, modalities, tools) onto OpenRouter's API shape.
• Surface OpenRouter's voice-capable models so @app.session(provider="openrouter", ...) can select them.

Acceptance criteria (behavior)

• @app.session(provider="openrouter", transport="audio_turns", ...) produces a working session against a real OpenRouter voice model (audio in → response), with the provider key staying server-side.
• The transport matrix only advertises pairs that have a real handler (no no-op entries) — i.e. either audio_turns works, or it's removed until it does (ties into Sessions: implement websocket transport + wire RealtimeSession.input()/say() live loop (currently scaffolding) #664).
• Tool calls route through execute/async with X-Session-ID, consistent with the OpenAI path.

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

[AbirAbbas]

Triage

Classification: feature ✓
Suggested labels: enhancement

Likely affected files:

• ./reasoners/models.py (matched: models, models, models, model, model, models, model)
• ./reasoners/repo_config.py (matched: config, config)
• ./reasoners/webhook_router.py (matched: route)

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Triage by github-buddy