* Python: Give the AG-UI Thread Snapshot lifecycle a single owner module
Both the agent and workflow runners independently implemented the thread
snapshot lifecycle: hydration replay, the load-once stored read, resume
message seeding, the stored/request/deferred-default state overlay, and
the save whose storage failures must never surface on an already-streamed
run. The two copies had already drifted in small ways (one hydrate helper
re-checked a store the caller had verified; the two cancelled-resume-id
helpers differed on missing-id handling).
Introduce ThreadSnapshotSession in _snapshot_session.py as the one owner
of that lifecycle, opened once per run and inert when no store or scope
is configured so callers stop branching on configuration. Rewire both
runners onto it, consolidate _cancelled_resume_interrupt_ids in
_run_common (defensive variant) and _event_messages_to_snapshot_dicts in
the new module, and delete the superseded per-runner copies. The session
interface is covered by dedicated tests; existing suites pin runner
behavior. Public exports are unchanged.
* Python: Narrow AG-UI event types in snapshot session tests
The hydration test accessed run_id, snapshot, and messages on values
typed as BaseEvent, which fails the tests/samples type checkers. Narrow
each event with isinstance assertions before reading its fields.
Evan Mattson
18997c2fde