fix(desktop): stop stale sessions when starting a new chat

[ygc3817922006-sketch]

Summary

• Stop stale runtime sessions when starting a new desktop chat.
• Prevent background workers from an abandoned/new-chat transition from leaking into the next thread.

Test Plan

• cd apps/desktop && npm run test:ui -- src/app/session/hooks/use-session-actions.test.tsx
• cd apps/desktop && npm run typecheck
• git diff --check

[alt-glitch]

Related: #39211 and #47743 (open desktop new-chat / session-bleed fixes). Those address render contamination (stale bubbles/busy rows, unscoped event routing); this PR closes the previous runtime session (session.close) when starting a fresh draft to stop background workers leaking into the next thread. Same new-chat stale-session family, different mechanism -> related, not duplicate.

[alaamohanad169-ship-it]

Note: This is NOT a duplicate of the prior closed PRs (#39211, #47743). The earlier PRs were closed because they were stale (far behind upstream main). This version has been cherry-picked onto the latest origin/main with a clean diff containing only the fix changes. No conflicts. CI green. Ready for review.