ocaml-ltp: enable MDX on eio/ltp_eio.mli
Run mdx on eio/ltp_eio.mli so the {[ ... ]} odoc block now type-checks
against the real Ltp / Ltp_eio APIs.
The example referenced free `session_id` and `process seg`, used the
`env#net` object-method shorthand, and silently swallowed an
unmatched send result. Wrapped in `let run () = ...`, gave session_id
a concrete `{ originator = 1L; number = 42L }` literal, switched to
`Eio.Stdenv.net env` with a `:> [`Generic] Eio.Net.ty Eio.Resource.t`
coercion (the sub-typing across closed/open polymorphic-variant tags
needs the explicit upcast), and printed the received segment via
`Fmt.pr "%a" Ltp.pp_segment` instead of calling an undefined
`process`.