fix(langgraph): checkpoint Topic as a flat values list (#2653)

Topic checkpoints now emit a flat values list like Python, so Host no
longer sees empty `__pregel_tasks` as `[[], []]` and fails
checkpointer_put. Legacy `[seen, values]` checkpoints still restore.

## Test plan
- [x] `pnpm test src/tests/channels.test.ts` (langgraph-core)
- [x] `pnpm exec vitest run src/tests/pregel.test.ts -t 'obtain correct
channel values|PregelExecutableTasks'`
- [x] Changeset added for affected package(s)