The proxy resolves its own public origin from PROXY_BASE_URL before it
looks at anything on the request, so a developer who has that set for
their own deployment watched 65 OAuth discovery, redirect_uri, and client
registration cases fail against an origin no test ever asked for
An autouse fixture now clears it for every unit test, matching the host
AWS config isolation that already sits beside it, and the tests that do
exercise a configured public origin keep setting it in their own body
