Fix rng gem

[ronaldtse]

The rng gem is meant to parse RELAX NG and RELAX NG COMPACT (RNC) schemas and provide a DSL for building them.

Currently the specs are failing.

The task is to create a fully working version of the gem that can read and round-trip RNG and RNC schemas. We need to add spec/fixtures files of known examples of RNG and RNC for round-trip testing purposes.

[ronaldtse]

The main issue now is the support for "blank XML elements" (like <text/>) that are heavily used by RNG to indicate presence. This was not supported by lutaml-model as we treated empty values practically as nil.

It is now supported with the new (and refined) value_map functionality (still draft):

• feat: added value_map option lutaml-model#357

[ronaldtse]

The failures are shown here:

• https://github.com/lutaml/rng/actions/runs/13734325743/job/38416157057