fix(session): ensure valid session ID is returned after model change

[avrum]

fix(main): fix model switching — duplicate events, broken sends, and lost context

Problem

Switching models in the model picker caused three issues:

1. Duplicate messages — every response appeared 2×, 3×, up to N× in the chat, worsening with each subsequent model switch
2. Broken sends — if the model switch failed, the tab's session ID became undefined and all subsequent messages failed with Session not found: undefined
3. Lost context — after switching, the new model had no knowledge of the prior conversation

Root Cause

PR #274 (a6f0230) refactored copilot:setModel to use client.resumeSession() instead of destroy() + createNewSession(), intending to preserve conversation context across model changes. Inspection of the @github/copilot-sdk source revealed this approach
was fundamentally broken:

1. resumeSession() does not pass the model parameter to the server (client.js:367). Only createSession() sends model in the session.create RPC call (client.js:307). The model was never actually changing on the server side.
2. Each resumeSession() call accumulates a server-side event subscription on the connection without releasing the previous one. The server sends N copies of every event after N resume calls — confirmed by logs showing event count grow by exactly +1 per
   switch (3×→4×→5×→6×→7×).
3. The session.destroy() call before resumeSession() was contradictory — it killed the server-side session, so the subsequent resumeSession() failed with "Session not found". After the failure, the session was already deleted from the local map, leaving
   the tab with a stale session ID. On the next attempt, the handler returned { model } without a sessionId, setting the tab ID to undefined.

Changes

src/main/main.ts

Model switching — copilot:setModel handler:

• Replaced resumeSession() with destroy() + createNewSession(). destroy() cleanly releases the server-side event subscription, and createNewSession() properly passes the model via the session.create RPC.
• Before destroying the old session, captures the full conversation history via session.getMessages() and injects it into the new session's systemMessage as a ## Previous Conversation Context section, giving the new model continuity. Gracefully degrades
  if message retrieval fails.

Event forwarding consolidation:

• Replaced 3 duplicate inline session.on() handlers (early startup resume, late startup resume, resumePreviousSession) with the shared registerSessionEventForwarding() function. Eliminates ~160 lines of duplicated code and ensures all code paths get
  consistent event handling — the inline handlers were missing session.error, session.usage_info, and session.compaction_* events.

Event handler lifecycle management:

• registerSessionEventForwarding() now returns the SDK's unsubscribe function (from session.on()), stored as unsubscribeEvents in SessionState.
• Unsubscribe is called during session cleanup: model changes, resumeDisconnectedSession(), and session destruction.
• Added a stale handler guard — the event handler verifies that the session object it was registered on is still the active session in the sessions map before processing events, as a safety net against leaked handlers.

src/renderer/App.tsx

• handleModelChange now validates that the response contains a sessionId before updating tab state, preventing the tab from being assigned undefined as its ID.

Testing

• npm run build ✅
• npm test — 384 tests passed ✅
• Manual verification: model switching works on first click, no duplicate messages after repeated switches, new model receives conversation context from previous session