[External] Copilot SDK: No OnDisconnected/OnConnectionLost callback on CopilotClient

[PureWeen]

Problem

When the TCP connection between CopilotClient and the copilot CLI server drops, the SDK provides no notification. PolyPilot only discovers the dead connection when SendAsync throws a connection error — at which point the user has already typed a message and hit send.

Impact on PolyPilot

• All in-flight sessions lose their event streams when the client is recreated
• Sessions that were mid-turn must be force-completed with partial/empty responses
• The user sees a brief Blazor surface reset as all sessions re-resume
• Observed today (2026-04-07): reconnect at 16:50 UTC caused 2 bystander sessions (Mobile-Fixes, maui-CIQuestions) to lose their in-flight responses

Desired SDK Behavior

CopilotClient should expose an OnDisconnected or OnConnectionLost event/callback. This would allow PolyPilot to:

1. Detect dead connections proactively (before the user tries to send)
2. Show a connection status indicator in the UI
3. Auto-reconnect in the background without disrupting the user

Current Workaround

PolyPilot catches connection errors in SendAsync, recreates the CopilotClient, re-resumes all sibling sessions, and force-completes any sessions that were mid-turn with dead event streams. This works but causes response loss for bystander sessions.

SDK Reference

GitHub.Copilot.SDK v0.2.1 — CopilotClient has no disconnect/connection-health API surface.