fix(frontend): refresh thread run list after stream so older history loads on active page

[64johnlee]

Problem

Closes #2965

After context summarization fires mid-session, scrolling to the top of an active chat page no longer loads older history. Reopening the conversation works fine, but staying on the same page leaves the older runs inaccessible.

Root Cause

useThreadHistory relies on useThreadRuns (query key ["thread", threadId]) to know which runs exist for the thread. This query is populated once on mount and never invalidated during the session. After a run finishes:

1. onFinish invalidates ["threads", "search"] (sidebar list) but not the per-thread run list
2. useThreadRuns returns only the initial (empty or single) run list
3. findLatestUnloadedRunIndex sees all known runs as loaded → returns -1
4. hasMore = false → scroll-to-top trigger is hidden
5. Older runs produced before/during summarization are unreachable until page reload

Fix

In onFinish inside useThreadStream, add one invalidateQueries call for ["thread", threadId]:

if (threadIdRef.current) {
  void queryClient.invalidateQueries({ queryKey: ["thread", threadIdRef.current] });
}

After the stream ends, useThreadRuns refetches the complete run list. useThreadHistory detects unloaded older runs, sets hasMore correctly, and the user can scroll-to-top to load them — matching the behaviour when re-opening an existing conversation.

Validation

• pnpm typecheck — clean
• pnpm lint — clean
• Normal (no-summarization) sessions: one extra runs-list refetch on stream end, negligible overhead
• Mock sessions: not affected (mock sessions don't trigger onFinish with real thread IDs)