Session_runner cleanup and webrtc refresh fix

[gtong-nv]

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[greptile-apps]

Greptile Summary

This PR refactors session execution into SessionRunner, updates callers and documentation, and changes WebRTC replacement and disconnect handling.

• Negotiates replacement peers before releasing the active connection.
• Adds a five-second reconnection grace period before ending a disconnected session.
• Refreshes browser playback and ICE-gathering behavior.
• Migrates runtime, tests, and the red-screen example to the class-based session runner.

Confidence Score: 4/5

The PR is not yet safe to merge because a reconnect near the grace-period deadline can still close the recovered session.

Disconnect cleanup can wake during the replacement offer's awaited SDP negotiation, observe no installed peer, and enqueue a session-closing event before the reconnect cancels the timer.

Files Needing Attention: flashdreams/flashdreams/runtime_v2/serving/webrtc_server.py

Important Files Changed

Filename	Overview
flashdreams/flashdreams/runtime_v2/serving/webrtc_server.py	Adds safe pre-negotiation peer replacement and delayed disconnect cleanup, but reconnect negotiation can race the grace timer and close a recovered session.
flashdreams/flashdreams/runtime_v2/session_runner.py	Refactors the module-level session loop into a stateful SessionRunner while preserving its threading, reset, backpressure, and cleanup behavior.
flashdreams/flashdreams/runtime_v2/serving/web/app.js	Waits for ICE gathering and explicitly starts video playback to improve browser connection and refresh behavior.
integrations_v2/red_screen/README.md	Correctly migrates the displayed integration example to construct and run SessionRunner.

Sequence Diagram

sequenceDiagram
    participant Browser
    participant Offer as WebRTC offer handler
    participant Timer as Disconnect timer
    participant Session
    Browser--xOffer: Existing peer disconnects
    Offer->>Timer: Start 5-second grace period
    Browser->>Offer: Submit replacement offer
    Offer->>Offer: Await SDP negotiation
    Timer->>Timer: Grace period expires
    Timer->>Session: CloseUserInputEventData
    Offer->>Timer: Cancel timer too late
    Offer->>Offer: Install replacement peer

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_{Reviews (2): Last reviewed commit: "Address greptile comments" | Re-trigger Greptile}

[greptile-apps]

Red screen example uses removed API

The red_screen README still tells users to import and call the removed module-level run_session function, so copying the integration example now raises ImportError; update it to construct SessionRunner and call run_session() instead.

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

[gtong-nv]

/ok to test ba01d36

[greptile-apps]

Reconnect races disconnect timer

When a browser starts reconnecting shortly before the five-second grace period expires, SDP negotiation can cross the deadline before _cancel_disconnect_timer() runs. The timer then observes that no replacement peer has been installed and enqueues CloseUserInputEventData, causing the session to shut down even though the replacement connection succeeds.