POC: Add Code Mode files tool golden tests

[lt-oai]

Why

Reviewers need to see that this is wired through the regular Codex Code Mode harness, not only the Responses API/Python path.

What

• Adds registry tests proving the files namespace is gated and handler-backed.
• Adds Code Mode exec-description coverage for the flattened runtime names: files_copy, files_export_for_tool, and files_materialize.
• Adds codex-tools coverage showing namespaced tools flatten into the Code Mode runtime shape.

Testing

• cargo test -p codex-tools namespace_tools_are_flattened_for_code_mode_runtime
• cargo test -p codex-core files_namespace_is_gated_by_experimental_supported_tool
• cargo test -p codex-core code_mode_exec_description_includes_gated_files_tools
• just fmt