Fix Pi feedback delivery after reload

[Whamp]

Problem

A Plannotator browser tab can outlive a Pi /reload. When its feedback arrives, the old callback correctly finds a newly registered Pi runtime, but current-pi-session.ts rejects that runtime as same-session because reload preserves the Pi session ID and file.

The callback then falls through to the stale captured pi object and reports:

This extension ctx is stale after session replacement or reload.

The notification fallback fails for the same reason.

Fix

Track the in-process extension runtime token as part of PiSessionIdentity. A reload now counts as a different active runtime even when its persistent session ID is unchanged. Existing session-ID/file comparison remains the fallback for identities without runtime tokens.

Test

Added a regression test that registers a replacement runtime with the same session ID and verifies both feedback and notification delivery go only to the replacement.

1 pass
0 fail
6 expect() calls

Focused command:

cd /tmp
npx --yes bun test /tmp/plannotator-debug/apps/pi-extension/current-pi-session.test.ts