Extract the ~16 standalone helpers (message reconciliation, optimistic/resolved
session upserts, stored-session resolution, runtime-info application, error
classification) out of the 1,254-line god hook into a colocated, tested
use-session-actions/utils.ts. index.ts keeps the hook orchestrator (the
stateful action callbacks) and consumes the helpers.
Pure restructuring, no behaviour change; folder index keeps the import path
(`@/app/session/hooks/use-session-actions`) intact. index.ts: 1,254 -> ~950.
Adds unit tests for the pure helpers.
ed47f2b4aa