🪲 BUG-#19: Fix stale process exit handler breaking provider/model switches

[FernandoCelmer]

Closes #19

reload() (called after switching provider/model, or any settings toggle) kills the old pycodeloop serve process and immediately assigns a new RpcClient to this.client. The old process's exit event fires later, asynchronously — its handler, still registered, wrote this.client = undefined unconditionally, clobbering the reference to the already-working new connection and posting a scary processExit ("pycodeloop serve exited (code null)") for what was actually just the expected teardown of the replaced process.

Symptom: switching provider or model shows a spurious error and the next prompt silently fails until reconnecting manually.

Fix: both exit and spawnError handlers now check this.client === client before acting, so a delayed event from a superseded client can't affect the currently active one.

Test plan

• npm test (43 passed) / npx tsc --noEmit / npm run lint clean
• No automated test — chat.controller.ts imports vscode directly, same limitation as every other file in this class (untestable under node:test outside the Extension Host)