Default durable threads to paginated history (#40677)

## What changed

When `thread/start` omits `historyMode`, select `paginated` for durable
threads whose store supports `thread/turns/list` and `thread/items/list`.
Continue to default ephemeral threads and threads backed by stores without
those APIs to `legacy`.

Explicit `legacy` and `paginated` selections keep their existing behavior.

## Testing

- Verify durable threads use paginated history with the local store.
- Verify ephemeral threads and stores without history-list support use legacy
  history.

GitOrigin-RevId: c94b6a3b522049d541da33c41618c4142370f1a7