Replace not-found exceptions in Arcadia find/show flows with typed outputs (Found*/NotFoundOutput) and map not-found results in presenters/responses while preserving HTTP 404 problem details.
Adjust architecture and unit tests to cover concrete output classes, and remove obsolete Arcadia not-found exception classes and their tests.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
