The custom JSON escaping of <, >, &, U+2028, and U+2029 existed only to
keep the encoded cursor byte-identical with the Cloud implementation of
the same payload format. Cursors are opaque tokens, so byte-level
compatibility across implementations is not needed — plain json.dumps
output is sufficient. Remove the escaping helper and the byte-identity
test fixtures that pinned the wire format; keep round-trip coverage for
the affected characters.