fix(ui): stop the create team form resetting organization and models (#39476)

* fix(ui): stop the create team form resetting organization and models

The organization preselect ran in an effect keyed on the organizations
query, so any refetch of that list while the Create Team modal was open
overwrote the user's organization pick, which in turn cleared their
models pick. The models field was also cleared whenever the available
models fetch resolved.

Preselect the organization when the modal opens instead, and clear the
models only when the user picks a different organization. An org admin
whose admin orgs narrow to one while the form is open can still pick,
rather than facing a locked empty field.

* fix(ui): block team create when the picked organization is no longer available

An organization picked in the Create Team form now survives a refetch of
the organization list, so it can outlive the admin's access to it. Refuse
the create with a message on the field rather than letting the request
fail authorization at the proxy.

* fix(ui): keep the team create organization field usable when the pick goes stale

Locking the field on a single admin organization also locked it while it
held a rejected organization, so an admin who lost access could not pick
the one organization left. Lock it only while it holds that organization.

* test(ui): hoist the created team fixture out of the mock call

The inline object pushed the repo past its no-large-inline-object-arg
lint budget, which has no headroom.