Use generated blank inputs to localize browser fallback errors

A generated blank media input can safely probe a typed server error contract without exposing real user content. Correlate the direct HTTP response with edge, application, transport, and engine evidence to determine whether the server returned a complete typed failure.

If the observed server response is complete while the UI shows a generic connection fallback, investigate the browser or request boundary—or a separate request—rather than rewriting the server contract. Route and hook tests are not end-to-end browser proof, and the exact browser cause remains unresolved until browser-side evidence exists.