ocaml-cookie: enable MDX on lib/core/cookie.mli, fix verbatim block
Add (mdx ...) stanza to lib/core/dune. The two odoc examples now use
Ptime.epoch as the deterministic clock and assert on the parsed cookie
fields (name, value, secure, http_only) so the build catches future
regressions in of_set_cookie_header / of_cookie_header.
In lib/jar/cookie_jar.mli, the Mozilla-format snippet was wrapped in
{[ ... ]} but is shell-tab pseudocode (not OCaml). Switched to {v ... v}
verbatim so it can be added to the mdx stanza later without false
parse errors.