[contextual_tasks] Add APIs for task and thread management.

This change introduces three new API methods to the
ContextualTasksContextController: CreateTask, AssignThreadToTask, and
UpdateThreadTurnId. These methods provide the necessary functionality
for creating new tasks and managing their associated threads.

Key changes include:
- Added CreateTask, AssignThreadToTask, and UpdateThreadTurnId to the
  ContextualTasksContextController and ContextualTasksService
  interfaces.
- Implemented the new methods in ContextualTasksContextControllerImpl
  and ContextualTasksServiceImpl.
- Added a new UpdateThreadTurnId method to the ContextualTasksService
  API to allow updating the conversation turn ID of a thread.
- Added unit tests for the new API methods in both the service and
  controller, including tests for observer notifications.
- Removed usages of RunUntilIdle in ContextualTasksServiceImplTest.

Bug: b/445841165
Change-Id: Ib0577aa35fbe656d020fe88ebfe67eca8eca9b9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7038467
Commit-Queue: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1529363}