sessions: multi-chat support for the local provider (#319315)

Local sessions can now host multiple chats. The parent->child hierarchy is
stored entirely in the provider's own IStorageService metadata via a new
`parentUri` field on the stored session, so it survives reloads with no
extra dependencies.

- getSessions() surfaces only primary chats; children are aggregated into
  their primary's group via a cached, live ISession whose `chats` observable
  re-derives on membership changes and whose status/updatedAt/isRead/
  lastTurnEnd are aggregated across the group.
- createNewChat(sessionId) on a committed session creates a linked child;
  sendRequest routes follow-ups (group id + child resource) to the child.
  A rejected/unsent child is rolled back and never persisted.
- deleteChat removes a single child; deleting the primary (or last chat)
  tears down the whole group. On load, an orphaned child (missing parent)
  is promoted to a primary.
- Multi-chat is always on (no setting).

Adds 10 unit tests covering grouping, persistence/restore, orphan
promotion, child/primary deletion, rejected-send rollback, and group
status aggregation. Updates LOCAL_CHAT_SESSIONS_PROVIDER.md.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>