Fix 1: OnSiteChoiceChangedAsync now resets _connectionChoices for every
RequiredConnectionMapping under the changed source site after loading the new
target's connections. Choices are re-seeded to the same-named connection on
the new target if present, or CreateNewValue otherwise — preventing BuildNameMap
from emitting MapToExisting for a connection absent from the newly-chosen target.
Fix 2: BackToUpload now calls ResetSessionState() before resetting _step so
_session, _preview, _resolutions, _siteChoices, _connectionChoices,
_targetSites, and _targetConnections are all cleared when the operator backs
out to re-upload, making it safe to start a new import flow from a clean slate.
Tests 12 + 13 added to TransportImportPageTests.
Joseph Doherty
f881521cc9