Gateway fail-closed privacy, sanitization, and cleanup

Treat provider-controlled data as untrusted. Upstream, malformed-input, and provisional-identity failures expose only canonical { type: "server_error", message: "Gateway request failed" }, upstream status, and bounded allowlisted retry metadata. Never relay, log, persist, callback, or attach to telemetry upstream bodies, status text, URLs, credentials, request content, arbitrary extensions, parser errors, malformed successful text, or private diagnostics. Public projections must be field-by-field and derived from trusted identity plus validated lifecycle state; unsuccessful callbacks contain only trusted identity, validated usage, and empty output. Before identity proof and durable commit, resolution content remains private. Read diagnostic bodies only under a bounded timeout, then cancel or release readers. Cleanup and telemetry must preserve already-emitted identity, settle exactly once, and never replace the original error.