Sessions: implement websocket transport + wire RealtimeSession.input()/say() live loop (currently scaffolding)

[AbirAbbas]

Follow-up from #654.

Two pieces of the session DX are intentionally scaffolding in #654 and should be tracked so they aren't mistaken for complete:

1. websocket transport is declared but not implemented. SUPPORTED_SESSION_TRANSPORTS lists openai: {webrtc, websocket}, but realtime-offer only handles webrtc (it errors on anything else). WebSocket is OpenAI's recommended transport for server-to-server realtime connections, so it's a reasonable next transport to actually wire.

2. RealtimeSession.input() / say() are placeholders. session.input() raises RuntimeError("populated by the AgentField control plane transport adapter") and say() just appends to an in-memory outbox. The handler-facing live loop (model audio/event → session.input() → handler → session.say() back into the session) isn't wired end-to-end; the validated path today is SDP proxy + tool routing. The PR body itself points at an earlier live spike rather than an in-repo end-to-end test for this.

Acceptance criteria (behavior)

• A session handler can await session.input() and receive a real SessionTurn populated from the live transport (not a RuntimeError).
• await session.say(...) produces audio/text back into the live session the user hears, not just an outbox entry.
• transport=websocket for provider=openai establishes a working realtime session (or is removed from the supported matrix until implemented, so the matrix doesn't advertise an unsupported pair).

This is a tracking issue for finishing the live handler loop; it does not block the SDP-proxy + tool-routing surface that #654 ships.

[AbirAbbas]

Triage

Classification: feature ✓
Suggested labels: enhancement

Likely affected files:

• ./reasoners/models.py (matched: model)

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Triage by github-buddy