Stop writing new session data back to the legacy tasks.session_json
column; updateTaskSession now upserts the active task_sessions row
(updating the session pointed to by tasks.active_session_id, creating
one when absent), and getTaskSession reads from the same row so
session data has a single source of truth. Adds migration 27 to
backfill pre-existing tasks.session_json values into task_sessions
idempotently via INSERT OR IGNORE guarded on task_id. The legacy
column is kept for read-only history and is not dropped.
ZacharyZhang-NY
a130dc29f5