test: cover sensitive data masking

[saurabhhhcodes]

Summary

• add Jest coverage for maskSensitiveData
• verify nested sensitive-key redaction, JWT/card-like value redaction, and non-sensitive value preservation

Fixes #1

Validation

• git diff --check
• node --check tests/core/masker.test.js
• npm test -- --runInBand tests/core/masker.test.js could not run locally because npm is not available in this shell

GSSoC labels requested

Please add/keep gssoc and the required difficulty/type scoring labels if this is accepted.

[saurabhhhcodes]

Closing this older version to keep the review queue clean. The current replacement for sensitive data masking coverage is #27.