[MB-0503][P1] Add unit tests for config, translation, and formatter/security modules

[FarisZR]

Summary

Ensure foundational utilities are well-tested before deeper migration work.

Scope

• Add tests for config loader, translation key handling, and output formatter/security helpers.

Implementation Notes

• Follow long-term modernization rules in docs/plans/2026-02-21-refactor-long-term-modernization-backlog-plan.md.
• Do not add short-term hardening if replacement is the target architecture.
• Keep changes PR-sized and reversible.

Acceptance Criteria

• Missing keys/config errors handled predictably.
• Escaping/sanitization utilities covered with edge cases.

Test Requirements (Mandatory)

• Unit test suite with invalid configs, missing translation keys, and injection payload fixtures.

Dependencies

• MB-0501,MB-0102,MB-0203

Definition of Done

• All acceptance criteria met.
• Required tests added and passing in CI.
• Documentation/config updates included where relevant.
• Backward-compatibility or migration notes added if behavior changes.

References

• Plan: docs/plans/2026-02-21-refactor-long-term-modernization-backlog-plan.md
• Backlog ID: MB-0503