Follow-up: harden translated toggle title fallback and regression coverage

[luandro]

Purpose

Follow-up from umbrella bug #157.

This issue tracks any remaining hardening work after PR #155 merges, especially around fallback behavior and broader regression coverage. It should be handled in later PRs only if validation shows gaps.

Scope

• document or implement expected fallback behavior when a translated toggle title is missing
• add regression coverage for edge cases around translated toggle section category generation
• confirm non-toggle translation behavior is unchanged

Checklist

• Define expected fallback behavior for missing translated toggle titles
• Add tests for fallback behavior if needed
• Add regression tests for translated toggle category generation edge cases
• Confirm non-toggle translation behavior is unchanged

Success Criteria

• Fallback behavior is explicit and tested
• Edge-case regressions are covered by tests
• Non-toggle translation behavior remains stable

Related

• Umbrella bug: Bug: translated toggle sections can keep English category titles in generated docs #157
• Implementation PR: fix(translate): use translated title for toggle section _category_.json #155