Follow-up trim of the salvaged #125414 commits:
- fetchHistoryWindow (GET /api/sessions/{id}/messages/around) is the same
session-scoped read class and now carries sessionReadOwnerPin too.
- client.ts: drop the unused mode field and the sessionOwnerForRead wrapper,
shorten the seam comment.
- sessions.test.ts: keep two invariant tests (owner pin applied; explicit
caller pin authoritative).
- use-timeline-history.test.tsx: stub sessionReadOwnerPin in its client mock
so the hook test compiles against the new import.
e55c1cb879