fix(desktop): stop pinning stale message at top on chat switch / compaction (#39959)

[victorftrdba]

Fixes #39959

Problem

After switching chats while a session is busy — or when automatic context compaction rotates to a successor session — the last sent message becomes pinned to the top of the viewport, and the pinned item changes while scrolling up. Classic virtualization row-reuse.

Root cause

• thread-virtualizer.tsx keyed the inner turn rows with the array index (key={index}), not the message id. On a session swap the new messages occupy the same indices, so React/@tanstack/react-virtual reuse the previous session's row DOM and state. (The outer virtual rows were already correctly keyed via getItemKey.)
• User bubbles use position: sticky (StickyHumanMessageContainer), so a reused/stale row is what visibly sticks to the top.
• <Thread> was never remounted per session — only passed sessionKey as a prop — so scroll-anchor / sticky state leaked across sessions.

Fix

1. Key inner turn rows by stable message id. turn groups now carry { index, id }[] instead of number[]; buildGroups populates both (iteration is unchanged).
2. Add key={threadKey} to <Thread> so it fully remounts on session switch/compaction, resetting leaked scroll-anchor and sticky state. AssistantRuntimeProvider stays mounted above it, so in-flight streams are unaffected.

Verification

• npx tsc -b in apps/desktop passes (exit 0).
• Manual: open two long sessions, send a message, switch chats while busy / trigger compaction → the correct session's user bubble sticks; no stale message pins to the top; scrolling no longer swaps the pinned row.