Follow-up on top of the helix4u #12388 cherry-picks:
- make deferred post-delivery callbacks generation-aware end-to-end so
stale runs cannot clear callbacks registered by a fresher run for the
same session
- bind callback ownership to the active session event at run start and
snapshot that generation inside base adapter processing so later event
mutation cannot retarget cleanup
- pass run_generation through proxy mode and drop stale proxy streams /
final results the same way local runs are dropped
- centralize stop/new interrupt cleanup into one helper and replace the
open-coded branches with shared logic
- unify internal control interrupt reason strings via shared constants
- remove the return from base.py's finally block so cleanup no longer
swallows cancellation/exception flow
- add focused regressions for generation forwarding, proxy stale
suppression, and newer-callback preservation
This addresses all review findings from the initial #12388 review while
keeping the fix scoped to stale-output/typing-loop interrupt handling.
4b6ff0eb7f