Prefer fail-closed validation over implicit fallback

When inputs, workspace discovery, parsing, or configuration are invalid or unavailable, preserve strict boundaries and fail explicitly rather than guessing, normalizing, or silently falling back to another behavior. Treat deployment prerequisites as intentional when they prevent ambiguous routing. Ensure parsing receives valid structured input and produces deliberate validation errors. Add focused regressions for boundary cases such as malformed JSON, unsafe paths, invalid identities/versions, and discovery failures.