Capture the parent artifacts directory and session ID when /tan dispatches
instead of resolving them through the mutable interactive SessionManager.
This keeps background tan '/data/workspaces/can1357__oh-my-pi__6971/.omp-session/2026-07-29T06-08-45-283Z_019fac7d-5ee3-7000-a7aa-16fe9394fdc9/local' reads pinned to the dispatching transcript
after the user switches or resumes another session.
Extend the regression test to switch the mocked interactive session before
the background job starts and assert the original local mapping is retained.
Fixes#6971
(cherry picked from commit e05db428eabd5087e4b2b4a462f47927c0628e72)
roboomp
fb4393ae16