Default notify to OWNER when creating WIP changes via REST API

When a change is created as Work In Progress (WIP) via Git push
(refs/for/master%wip), ReceiveCommits defaults notify handling to
NotifyHandling.OWNER (unless overridden via %notify=...). This ensures
that project watchers are not notified about WIP changes before they are
ready for review.

Note that WIP changes differ from Private changes: Private changes rely
on visibility ACLs (users with VIEW_PRIVATE_CHANGES permission still
receive notifications as tested in watchProjectNotifyOnPrivateChange).
WIP changes, on the other hand, are visible to everyone with read access
but suppress notifications by defaulting notify handling to OWNER.
Users added as reviewers or CCs on WIP changes also do not receive
project watch notifications for those WIP changes unless notify=ALL is
explicitly specified.

However, when a WIP change was created via the REST API (POST /a/changes/),
ChangeInput had a field initializer defaulting notify to
NotifyHandling.ALL, and CreateChange unconditionally fell back to
NotifyHandling.ALL. As a result, WIP changes created via REST (such as
via Cog or automation tools) dispatched email notifications to all
project watchers by default.

Fix this by:
1. Removing the default field initializer from ChangeInput.notify so it
   is null when unspecified by the caller, and updating
   ChangeInputProtoConverter to handle nullable notify.
2. In CreateChange, defaulting notify handling to NotifyHandling.OWNER
   for Work In Progress changes (matching ReceiveCommits and
   CommitUtil.createRevertChange), while keeping NotifyHandling.ALL for
   ready changes.
3. Updating the ChangeInput entity documentation in
   Documentation/rest-api-changes.txt to reflect the contextual default.

Add acceptance tests in ProjectWatchIT verifying that project watchers
(including users added as reviewers or CCs) do not receive notifications
for WIP changes created via REST, updated with new patchsets, or pushed
without notify override, while still honoring explicit notify=ALL.

Forward-Compatible: checked
Google-Bug-Id: b/542620083
Release-Notes: Suppress project watcher email notifications on REST-created Work-In-Progress changes
Change-Id: Ic97c7d8c9de5adcdd54475e3869d2a35aeab5736