PR #16013 plugged the leak in `/new`, but two sibling session-boundary
resets had the same bug:
1. Inactivity / suspended-session auto-reset (top of `_handle_message`)
previously cleared only reasoning. Now drops model override and the
queued "/model switched" note as well.
2. Compression-exhaustion auto-reset now also drops the pending note
alongside the existing model/reasoning cleanup.
All three session-boundary sites now use the identical cleanup idiom.