test : exported digest-email helpers and added unit tests

[tmdeveloper007]

Closes #2812.

Summary of What Has Been Done: Created test/digest-email.test.ts with 25 test cases covering esc, pluralise, and langColour. Also exported these helpers from src/lib/digest-email.ts and added a String() coercion to esc to handle non-string inputs safely.

Changes Made:

• src/lib/digest-email.ts: Added export to esc, pluralise, langColour functions; added String coercion to esc to prevent TypeError on non-string input.
• test/digest-email.test.ts: 25 test cases covering XSS character escaping, pluralisation logic, language colour lookup.

Impact it Made: esc is a critical XSS sanitiser for the weekly digest email. Making it defensive (coerces non-string to string) prevents crashes and hardens the email template against unexpected input types.

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

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• level:beginner — 20 pts
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• level:advanced — 55 pts
• level:critical — 80 pts

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