fix: add collection: true to mixed content attributes

[ronaldtse]

Summary

• Add collection: true to 58 xhtml model classes that use mixed_content + map_content without collection on the content attribute
• Required by lutaml-model's MixedContentCollectionError validation
• Update test expectations to expect arrays instead of strings for content attributes
• Run rubocop -A --auto-gen-config to update rubocop todo

Test plan

• CI passes across all Ruby versions