Snapshot this.ctx.sessionManager.getSessionId() for the tan clone's local://
mapping instead of session.sessionId. The two diverge after /fresh or a
provider session override, and the Windows short-root fallback keys
%TEMP%/omp-local/<id> off the session-manager id used by the parent's
large-paste writes and '/data/workspaces/can1357__oh-my-pi__6971/.omp-session/2026-07-29T06-08-45-283Z_019fac7d-5ee3-7000-a7aa-16fe9394fdc9/local' reads, so the mismatched id left attachments
unreachable.
Diverge the mocked session id from the manager id in the regression test so
it pins the session-manager id.
Fixes#6971
(cherry picked from commit 1efcd22326d76fdb8b50c0977a836c892e80ab76)
