[codex] Let hide_fields override show_fields

[xantrac]

Summary

Aligns show_fields with the merged hide_fields runtime visibility implementation from #442.

• Allows a field to appear in both show_fields and hide_fields.
• Keeps the existing show_field?/2 precedence where hide_fields wins.
• Updates DSL docs to state the precedence explicitly.
• Adds runtime, OpenAPI, and JSON Schema coverage proving hidden fields remain hidden even when listed in show_fields.

Validation

Passed with local available toolchain ASDF_ELIXIR_VERSION=1.19.0-otp-28 ASDF_ERLANG_VERSION=28.1:

• mix spark.formatter
• mix format --check-formatted
• mix compile --warnings-as-errors
• mix test test/acceptance/field_visibility_test.exs
• mix test test/acceptance/open_api_test.exs
• mix test test/acceptance/json_schema_test.exs
• mix test test/acceptance/get_test.exs test/acceptance/index_test.exs test/acceptance/get_related_test.exs test/spec_compliance/fetching_data/inclusion_of_related_resources_test.exs
• mix docs

Full mix test was also run, but it currently fails in existing pagination tests with a Xema/Regex exported pattern error under the local Elixir/Erlang versions. The repository-pinned patch versions were not installed locally.