Adds TestErrorMatrix (23 parametrized rows) covering every exception class
raised or mapped in files.py and app_run.py, asserting the exact wire code
each path emits and that every emitted code is an OpenApiErrorCode member.
Also adds error_code = "filename_not_exists" to FilenameNotExistsError, which
had no explicit code and was falling through to the status-map (bad_request).
