fix: reduce basic-auth false positive for markdown mailto badges

[AchieverSana]

Summary

• Fixed generic-basic-auth-url false-positive on markdown badge links followed by a mailto: link on the same line (e.g. shields.io badges).
• Excluded ] and ( from the username/password character classes so the match can't span across markdown link syntax.
• Added positive/negative test cases in tests/test-patterns.js.

Type of Change

• Pattern improvement (reduces false positives/negatives)

Checklist

• I have read CONTRIBUTING.md
• Pattern validation passes (node scripts/check-patterns.js)
• Tests pass (npm test)
• No real secrets are included in this PR
• Documentation is updated (if applicable)
• I understand maintainers may require CODEOWNER review before merge

Test Plan

• Ran node scripts/check-patterns.js — all 78 patterns valid, 0 errors
• Ran node tests/test-patterns.js — 51 passed, 0 failed, including the new false-positive case
• Verified https://admin:S3cr3tP4ss@internal.example-corp.com/api is still correctly flagged as basic auth

Related Issues

Fixes #26